Dragon Age: Origins

This is the quickest way, bar none (assuming you have the Ultimate Edition or if you've bought the Feastday Gifts dlc). I got this as my last romance plot and skipped the cutscenes and basically ran as fast as I could to get to Camp... Took me around thirty minutes after beating the origin story. Super Easy.
Good luck!

*** If you aren't able to buy the feastday gifts, then this guide will not work for you:

1. Play through a female human noble origin story. ***female dalish elf, female city elf, female magi and female dwarf noble are all confirmed to work as well. Thanks to everyone in the comments for the confirmations! (If you know of another race that will work, please leave a comment. Thanks!)

2. Continue playing through Ostagar (just run through missions, keep going towards the white arrow on the map).

3. Get to Lothering and go to the arrow on the map to save Bodahn Feddic.

4. After saving him, run towards him to trigger a cutscene and speak with him. Afterwards, run opposite towards the World Map arrow.

5. Go to the camp and SAVE.

6. Find Bodahn at your camp and pick to see his wares.

7. Go over to the "other" things he has and scroll down to buy a "Grey Warden Hand Puppet" and five "Thoughtful Gifts" - All of this will cost 75 silver. Just sell everything you've picked up along the way and you should easily have enough money.

8. Give all of these to Allistair and it will max out his approval rating.

9. Speak with Alistair at camp and choose the following dialog options:
"has anyone ever told you..."
"you know you're handsome..."
"that would be nice"

10. Speak with him again and choose:
"miss the constant battles..."
"I think I already do"
"I don't know. I need..."

11. Speak with him one more time:
"I'd like to discuss..."
"So how would you like to join me..."
"If you don't want to..."
"I think this is as right as it gets"
"Just follow my lead Alistair"

12. After the cutscene, achievement unlocked.

This is much easier to do with a female character, although I haven't had the pleasure of trying to woo Alistair with a male character. At any rate, this is easiest to do with the knowledge that even though Alistair does not say it, he is very religious and highly moral. Taking the high route may not always increase Alistair's approval rating, but doing something morally abhorrent will most certainly decrease it. Alistair will not sleep with your character until he has an approval rating that is nearly full. You should be in the last "bar" on the approval scale before asking him to sleep with you. To get Alistair's approval rating that high, you need to make sure to do the following things:

1. Always take the high moral and/or religious road (as stated earlier)
2. It would be in your best interest to save Arl Eamon instead of defiling the Urn of Sacred Ashes-- choosing to defile the ashes may result in Alistair leaving your party, I'm not sure, as I have not done so yet.
3. During the crisis at the castle involving Isolde and Connor (The Arl of Redcliffe/Urn of Sacred Ashes quest) Alistair will very much disapprove if you kill Connor or allow Isolde to sacrifice herself for Connor. Your best course of action would be to request assistance from the circle of magi (if you have completed broken circle quest line and saved the mages). If you chose the templars, your next best course of action would be to allow the blood mage Jowan to perform the ritual to save Connor's life in the fade by sending in one the mages from your party, or yourself if you are a mage, to do the deed. I would suggest Morrigan over Wynne though, because Wynne will disapprove of being involved with blood magic whereas Morrigan will do it with no complaints. (PLEASE NOTE: In my game, I killed Connor without allowing for the ritual to be performed, and this caused Alistair's approval rating to drop 20 points. I still got this achievement though by showering Alistair with gifts)
4. Do Alistair's side-quest. To do so you need to keep talking to Alistair until his approval rating is high enough that he tells you about his past and how he wants to meet his sister Goldanna in Denerim.When you get to Denerim you meet his bitch of a sister, who will cut Alistair down for being successful while she is sitting in poverty with five children. She makes an attempt to extort money out of Alistair and your party. I believe giving the money to Goldanna will increase Alistair's rating (I do not know if refusing to give the money will affect the approval rating). After the encounter with his sister, you need to be there for moral support for Alistair, and you can do so by either consoling him or hardening him from the experience. Completing this quest will make romancing Alistair much easier.
5. Romance Alistair with many gifts, especially his mother's locket which is inside the Arl of Redcliffe's desk inside of the castle. Alistair also enjoys the dragon statuettes, found all over Ferelden.
6. During camp scenes, do not pressure Alistair to have sex more than once. If he declines the first time and you are understanding, his approval rating will increase. However if you continue to pressure him after this conversation his approval rating will decrease. Avoid discussing sex with Alistair until his approval rating is nearly full.
7. During conversation Alistair does not like to be mocked and does not like to be given tough choices to make. Be a level headed leader and flirt with him but avoid mocking him and your conversations will go smoothly. Avoid saying anything bad about Duncan.


Also it is possible to romance Alistair even though you have already romanced another character in your party, (such as Zevran, Leliana, or Morrigan). In my game I romanced Zevran first, and after awhile, Alistair asked me to choose between he and Zevran, in which I chose Alistair, which followed up with a rather graphic steamy scene between Alistair and my main character in her tent.

You can actually get this fairly quickly, like.. not completing any of the after-Ostagar main storyline quickly. I managed to get it about 3.5 hours into the game (counting origin story, Ostagar, etc.,) obviously without having to get Duncan's Shield or his mother's locket. It takes a little bit of effort, but is totally doable. Make sure you are a female character, because I am fairly sure that is the only way to do it. Here are the basic steps I took to get the achievement early in the game.

1. Make sure you get Dog, even if you are just doing a playthrough to get this achievement and then you are done. Dog can get you a few gifts that will help improve Alistair's approval rating when you ask Dog to look around, and every point you can get is important if you are just trying to get this achievement or the "Hopelessly Romantic" achievement.
2. Save any and all gifts that you find in Ostagar or Lothering to use on Alistair, and make sure you use the gifts that he really likes (statuettes, etc., as the other user posted) first.
3. Talk to him as much as possible. Sympathize with him after Duncan dies and you awake at Flemeth's Hut. After sympathizing with him, you can immediatly begin conversation with him that will improve his approval rating. Make sure to mention how much you liked Duncan and how you can understand how Alistair feels, and that you'd like to help. Eventually it will get to the point where you can call him handsome. If you are having trouble getting new dialogue options with him, give him the gifts you have saved to use on him. Make sure you take up every possible dialogue option, and use the obvious "nice" or "flirty" options. The various conversations will at first include his talking about (in the order as best my memory can serve) Duncan's death, then questions about the Grey Wardens, then talking about his training as a Templar, then talking about what it was like when all the Grey Wardens were around, then talking about his life in the Chantry and how he became a Grey Warden. More dialogue options will open as his approval rating rises with good responses to the other conversations + gifts.
4. When I did this, I made sure to go to every possible place on the map that I could buy a gift even if it was only a +1 gift. Places I went to buy gifts: The vendor at camp (you can actually buy his gift twice if you leave camp and come back), The Gnawed Noble Tavern, the tavern at Lake Calenhad Docks, Varathorn at the Dalish Camp, Wonders of Thedas shop in Denerim. Those gifts, coupled with the gifts you find in Lothering and Ostagar (be sure to look around), and the ones that Dog finds you should be enough (with talking) to get his approval high enough.
5. Also make sure to go to the Village of Redcliffe, and tell him you understand his decision to hide his lineage, this will get a few more approval points. It will also open up new dialogue about why he hid his ancestry, continue to empathize with his decision and you will get a few more points (something like +4 total). You do not have to complete any of that quest, just going to the village prompts his dialogue, and you can leave immediatly after.
6. Eventually, he will mention his sister, Goldana in Denerim, after you have talked to him about everything possible and probably used all or most of your gifts. Taking up the quest will give you some approval points, and completing it and comforting him after his sister turns out to be a bitch gets you quite a few points. After I did this quest and returned to camp, he first simply spoke with me and thanked me and kissed my character, gaining further approval points. I talked to him immediatly after that and was able to convince him to come to my tent.

NOTE: Be sure and save before you proposition him to come to your tent, because if your approval rating is not quite high enough you will actually lose approval. If for whatever reason, after completing his Companion quest he still says no, go to any vendors you haven't gone to yet and buy gifts. He is more than likely very close to agreeing to come to your tent, and +1 or +2 may be all you need. Even though I didn't go, there is probably a vendor or two in Orzammar who you can buy gifts from, in the event that you don't quite have the approval rating you need. Message me if you have any questions.

Hope this helps out!

OK, The situation i was previousley in required me to start a fresh playthrough. That is if you were, Of course stuck at a dead end with alistair. If this is the case and no other solution works then start a new playthrough. Yes as daunting as this sounds it's worth it in the end.

STEP 1 - when you first meet alistair and become a grey warden, you'll encounter a town called lothering slightly after the ritual. Here you'll want to take all the morale choices that will make alistair turn in your favour. also whilst you're here there's a few gifts that can be bought from the merchant that will come in handy later.

STEP 2 - Head to redcliffe and you should encounter a cutscene with alistair, Be as nice as possible with him as this triggers his side-quest when you return to the party camp. Head to the castle, here you're given the choice to either kill conor (demon kid) or sacrafice his mother, Neither of these are neccesary as you should take the third option which is to get lythium from the magi tower. This will result in alistair's approval rising up. (the other 2 choices have negative effects).

NOTE: - whilst at the castle grab alistairs mothers amulet as it increases his like by 12+ further on.

STEP 3 - Return to the camp with as many gifts as possible obtained from redcliffe and lothering. (i only ever took/bought gift's from these two towns and still got to sleep with him). Here he will speak to you about his past as a child and tell you he never got to meet his half-sister. Your task now is to simply head to denerim. Head towards the market and you should come across a cut-scene. he'll inform you that the house next to wade's imperium is his sisters. follow this dialogue and you should get 5+ approval when you exit the house.

(in the house)That's hardly Alistair's fault, is it?

(outside)
-You don't need her. You have others who care for
you.
-I care about you.

After this head back to camp.

STEP 4 - now save. speak to alistair and activate all possible dialogues that will further his approval/affection. If you feel you've done the best possible for leeching his affection then SAVE. Now if i'm correct, ask to speak to him in person and the option (kiss) will appear. Do this. After that back out of the dialogue with him then re-enter. He should then give you a rose. Again be gratefull with him and he'll try changing the subject. Back out of the dialogue again and SAVE. This time ask him if he'd like to accompany you to the tent and he should be shy/nervous. After a few sentences he'll aprove and gladly enter the sex scene. Now you should get that annoying 10g.

If you need any help message my gamertag.