BioShock

This one is simple. Make sure that you either rescue or harvest each Little Sister on every level. It doesn't matter whether you harvest, rescue, or any combination of the two, as long as you deal with each one.

Medical Pavillion - 2
Neptune's Bounty - 3
Arcadia - 2
Farmer's Market - 1
Fort Frolic - 3
Hephaestus - 3
Olympus Heights - 2
Apollo Square - 2
Point Prometheus - 3

Quite easy actually, the hardest part is finding/killing all the big daddies.

If you don't know if you've dealt with all of them in an area just pause, there will be an icon at the bottom telling you how many are in an area, the icon will be different whether you've harvested, rescued, or not dealt with the sister. If a big daddy is in the icon you haven't dealt with it.

When you find them there are some easy ways to kill the big daddies.

1) Get the hypnotize big daddie plasmid from Tenenbaum by saving some little sisters. Then find a big daddy hypnotize him and lead him to another. Attack the second and then the 2 big daddies will fight until one is dead. If it's the one you attacked then you will have to finish the job. And if it's not then the second will be an easy kill. Not only is this a good way to kill the big daddy, it is one of the most awesome minions you will ever control, and the fight is also awesome to watch.

2) Get the security bullseye plasmid (forgot how). Find a big daddy and see if you can find a camera. LEAVE THE CAMERA ALIVE, DON'T HACK IT. Then you can use security bullseye on the big daddy alert the camera and all the security bots will go after the big daddy leaving you alone. Keep repeating until the big daddy is dead. The good thing about this method is the big daddy will leave you alone after the alarm is over/

3) Equip Static discharge and stock up on electric buck and trap bolt/steel bolt. Lay some trap bolts with your crossbow spread a good distance apart that the big daddy will run into. Stay on the opposite side of your trip bolts and alert the big daddy, then try and use your crossbow and kill him before he gets to you, if he does get close use your electric buck. Shoot and then wait for him to start moving and shoot again this will keep him stunned and may give you enough time to reload a shell. Hopefully he dies before he gets close, but if he does hit you static discharge should stun him.
EDIT: This tip is mainly for the "Drill" big daddies, for the ones with the rivet gun you should use one of the other tips.

Some general tips:

1) ALWAYS make sure the little sister is with the big daddy, otherwise you have no real reason to kill it.
EDIT: If you see a Big Daddy without a little sister, but haven't gotten all of them in the area follow the Daddy to a vent and there is a chance he will call a sister from the vent. Leave some distance when you're following it, if you're to close he will not grab the sister.

2) The crossbow does a lot more damage than you may think and is actually your best bet at long range, use it to your advantage. Not only that but the trap bolts will also do massive damage and will stun him for a short time.

3) ALWAYS do your research photos before you kill it. You can take several photos and if he's near the little sister you can get close for an action shot/multiple subject bonus. The research will drastically increase your damage done.

4) If you have completed your research equip the plasmid that increases research damage. (this will help with any other enemy you have researched as well.)

5) Remember to pause and make sure you've dealt with the little sisters in the area before you leave. Although you can come back later it will be hard to remember which areas to go back to, and dealing with them while your there is much easier.

This isn't so much a total solution as a great recommendation. The one thing I learned to help defeat the Daddies is to have them fight themselves. Be sure to get the Hypnotize Big Daddy tonic - it is given to you from Tenenbaum after saving only a few little sisters. While you have one Daddy hypnotized and by your side, find another with a little sister, shoot him, and then run away. The two will fight till one dies - you can stand a distance away and finish them both off with heat seeking RPG's or Electric Gel. You get the loot off of both and the little sister without too much effort of a Daddy battle. If the one you attacked survives the battle, be sure to finish him off - he will come after you. His health will be low so you can kill him pretty easy.

Well to do this you must defeat every Big Daddy (Actually, only the ones with Little Sisters with them) and by pressing start, you will see icons on the bottom of your screen, they reflect how many Little Sisters are left, and how many you've saved/harvested. But for the real advice, Big Daddy hunting. First off, if you find a Big Daddy without a Little Sister, but there are Little Sisters left in the level, you must follow the Big Daddy and wait for him to hit one of the tunnel holes on the wall, but the first thing your going to want to do is prepare for battle. First thing, save before setting traps in case things go wrong, after that check how much "Trap Bolt" Ammo you have for the crossbow, then how many "Proximity Mines" you have (Grenade Launcher) then start setting traps, make sure the trap bolts are connecting a straight line so it is easily walked into, then make sure it's about chest level, so the Daddy's can actually hit them, and for the Proximity Mines, just make sure you set them withing reasonable distance to a hopefully, still Big Daddy (Paralyze or freeze him first if needed) Also, you can put them around the trip wires and it's almost a sure bet he'll get hit by both. So with your traps set, bring out your Crossbow or Shotgun (Machine Gun with Armor Piercing Rounds work great as well) And if he isn't picked off by the traps, just shock him, then hit him with your weapons, he'll go down easy.

There is a wrinkle to this achievement that will make you sweat if you aren't aware of it. Pausing displays icons to tell you the number and status of the little sister's in the current level, however, you can sometimes rescue one more little sister than the game says. I managed to rescue 4 little sisters in Neptune's Bounty although the game says there are only 3. The wrinkle is that when you try to leave the level, a message is displayed saying that you haven't dealt with all of the available sisters. You can ignore that message. Apparently the programmer coded "equal to" when he/she should have coded "greater than or equal to".
Interestingly enough, you will get the achievement when you rescue 21 little sisters. So because I rescued one extra little sister, I got the achievement even though I had one more little sister left in the level. Rescuing the 22nd little sister appears to have had no unfortunate side effects, and I was rewarded with more Adam.

Hi
location of the little sisters :

(I'm not the author of the video)