I usually never raise more than 3x the big blind unless the person re raised me. I find out my own style of playing is prett effective and i've been really successful in tournament and cash games.
3x is pretty standard; you are increasing this if you have limpers in ahead of you though, right? If you have two limpers ahead it should be 3x + 2 = 5x, otherwise it's just a pot sweeting raise which is fine if you have AA.
If the stacks get small relative to the blind size, which happens in turbos, going down to 2.5 or even 2 can be fine.
Also, Phil Gordon made a good point in an article I read somewhere on the internet, about sizing your raises according the stack sizes behind you. If you. For example, you have a small stack with 4BBs in the blind but a big stack with 15 BBs in the small blind. With a hand like KJ you don't necessarily want to tackle the big stack but you'd be happy to get the small stack all in, so make a raise like 2.5 that targets the small stack, but lets you fold if the big stack reraises.