WATCH_DOGS

EDIT Aug 26, 2014: The recent patch allows you to hack/tail your friends. These tails do count towards the achievement, so it's easy to boost now. Just make sure you do NOT scan or kill your friend - they must successfully tail you for it to count. Thanks to those in the comments for confirming this for me.

Patch notes: http://forums.ubi.com/showthread.php/911148-Patch-Notes-08-2...

Old solution below, for those curious what sparked both the love and hate in the comments:

For those who want to force this to happen quickly, here's a pretty good strategy. I don't know why it works, but it does work.

Step 1: Go to options -> online -> turn off invasions. This will RESET your notoriety (MP experience points) to zero. If you don't want to lose your notoriety, don't use this solution - try one of the others.

Step 2: Turn invasions back on.

Step 3: Do an online tail of someone else. It doesn't matter if you win or lose. For me, I wanted to have fun and usually won. In the comments, people are reporting more success if they intentionally lose. Losing is definitely faster.

Step 4: As soon as you're back in your world, you'll get invaded. It happens within 10 seconds without fail. You can tell you're being invaded because if you pause the game, things still keep moving in the background. Most of the time, this will be a hack, and those don't count. Do what you want with these - find them, ignore them, whatever.

Sometimes, during step 4, you'll see that you're being invaded (pause menu trick) but you'll NEVER get notified about an attacker. That's what you want. After a few minutes of this the invader will leave and your achievement progress will increment.

Regardless of which invasion you got in step 4, repeat at step 1. Keep doing the off/on/invade/be-invaded rotation until you get the achievement.

I do not understand why this works. I know for a fact that when I tail other people, they get notified that they have to find me. They start running around like crazy, so they must know. Sometimes when I get notified that I'm being invaded, I know I'm being tailed. The game notifies me. Why does this achievement only tick up if you get no notification at all? Who knows. Maybe it's bugged and it only ticks up if the invader quits out early (they can do this without penalty, as long as they haven't started the hack).

I FOUND A EASY SOLUTION WITHOUT ANY EFFORD OR LOSING YOUR RANK....!!!

just start up the game : play a tailed online game and LOSE !!!
leave your controller on WITH a play and charge kit and rubber band your analogs so you dont log off for inactivity....
just go to sleep or watch movies do what ever you want ...
i went to bed for 6 hours and got tailed 4 times...
i thingk you need a bit of luck but just give it a try when going to work or sleep and when you get back you schould got progress or the achievement :)
Good luck hope this helps !

Ty dillionday for this tip : (this doesn't seem to work for everyone...i just used my method above and got free tailed 4 times in few hours ...)
After you win/lose an online tail immediately go to a drinking game or a city game like find the ball. Leave it on the bet screen and wait. This will keep the fixers from stealing your data but fixers will still be able to tail you. You won't know because the game won;t tell you while you are on the betting screen.

This is the opposite to the Piggyback achievement. 5 players have to follow you around while you free roam the city. This being said, online must be enabled.
Do not play missions or mini games - gather the collectables or mess around instead.

Tips:
*Finding the enemy fixer and stopping the tailing does NOT count! - Thanks Spartan8722
Update: Some players say it will count, if the enemy fixer is killed. I can not confirm this - at least for me it didn't work. Feel free to kill the intruder but check your progression regularly. If you killed someone and it shows no increased percentage just let them live and don't disturb them.
*You can check your progression with your Xbox achievements app. There is only one hidden achievement with a progression bar.
*Playing online - especially the tailing - should increase your chances to become the victim.
*There is NO notification when being tailed unless you try to start some sort of mission. When you try it tells you to find the enemy fixer and supports you with the purple circle.

TRY THIS IF YOU DO NOT WANT TO RESET YOUR REP


**Edit** I should also note the online is not completely available until you have an unusual meeting with ****, and your second branch of skill tree is unlocked. After you regain control of Aiden, press start and quit mission, online should be completely unlocked.
** Edid2** Definitely must be tailed. The only way to know if you are currently being tailed is to try a mission, you will be notified that you must stop the tail to start mission. The yellow Y marker will turn purple. With that said if you notice npc's acting weird, walking crazy, or running ignore them. Stop moving or go the other way. Do not look at them or do anything to spook them. Pretty sure they need to "stealth cookie" you for you to get credit for this achievement. Tailing contracts will also increase your chance of getting tailed.

Like Tenreth said, online must be enabled but you also need to make sure you are connected to ubisoft servers as well. The only way I found to check this is to go to online contracts and request a tail mission. It will start to search within a few seconds to 20 a msg may pop up saying you are not connected. If it does, quitout to main menu, dashboard, press start to quit game ( or whatever button they are calling it now). I had to do a hard reset on my xbone everytime it told me ubi servers are not available. To hard reset just hold the Xbox light on the front of your console for 5 to 7 seconds, it will power off. Wait a min then turn it back on. You can tell it did a hard reset if you get the green Xbox screen after console starts to boot up.

Now for me I had 60% completion before I did this method. Only knew of one tail I lost, because it popped up after he escaped. Lost a few hacks as well but that was it. I busted countless people hacking me, before they initiated and when they were like 98%. several tails as well. Which means I was unable to let people win knowingly, just couldn't help myself. So if your like me just walk away, and let it do its magic.
I went to a train station on the far side out in the open, hit x make sure phone is active, face the camera down to your feet, so you can't see any body around no profiling nothing. Plug controller in and walk away. Leave the tv on the game, don't turn it off or change channel. When ever I did this I would come back and there would be a msg about my controller and game was essentially paused. I wold check and I was not connected to ubisoft. Hence the hard reset. Did all this about an 2hours ago, left the tv on with the game onscreen, and did my Friday grocery trip. I came home to find a shiny new achievement. The screen went dark but the game did not pause and no controller msgs. Tried a online contract and connected to a match almost immediately. You MUST be connected to ubisoft servers for people to invade you. Also not certain if hackers that win count or if it has to be tailed. Do this before bed or a trip to town and you should have made some progress or gotten the achievement. As long as everything is how you left it beforehand, no controller or ubi server msgs. This is the least painful way I found.

One last thing before you set this up, pause the game an quit any mission you have active, if the option is grey and you can't select it, you are good.
Hope this helps, let me know if you have anything else to add.

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This video shows how you get the achevement. It is much faster done with a friend as you could be waiting hours,days or even weeks just waiting for others to tail you.
Make sure you dont profile the tailer or it wont count towards your progress.
When 5 have been done > achievement unlocked!