I think it depends a lot on who the limpers are. I love to attack limpers, so I take detailed notes about what kinds of hands people limp, how much it takes to get a fold from them, how they respond to a continuation bet, etc.

The loosest people limp any two suited cards, any two connected cards, any A, and sometimes random other hands like K9, Q8, etc. Generally these players liberally call raises and play poorly after the flop as well. If you get the opportunity to isolate them with a hand like KJ or AJ (which could mean that there are other limpers but you think the others will fold to your raise), go for it. You should usually continuation bet in this spot no matter what, since it is so unlikely that the board hit the limper, but if he calls, you're usually done with the hand if you don't improve to better than top pair. Sometimes you end up giving free cards to a flush draw or folding to a river bluff from a busted draw, but on the whole, this is a profitable situation. It's a move best made in position. I like to be on the button or the CO, very rarely will I do this from the blinds. Also, suited are better than non-suited cards for this purpose, since your flop bet will usually allow you to see the river for free.

Some players limp weak to medium strength A's in any position, even on the button. Assuming I can get heads up with them, I'll raise these guys if I've got AT+, maybe even A9s, and position. I'll raise AJ+ from the blinds against them. When you hit top pair, you've got to value bet against these guys all the way. They will usually call down aces and a worse kicker, and if there aren't a lot of scary cards on the board, they'll even call down if they just paired their kicker. This also means you can't continuation bet very effectively against them. Usually, they only have three outs against you, so you don't have to worry too much about giving a free card, but they are not going to call a bet unless they've paired their kicker, and then there is no getting rid of them. I'd rather take a cheap showdown.

This all assumes you have a good read on one particularly loose player and you think you can isolate him (which requires having good enough reads on the other limpers to know they'll fold).

If you don't think you'll be able to isolate a weak player in position, then you should throw away hands like AT and AJ that don't play well in multi-way pots. I'd still raise AQ, but I don't hate calling with it either, as long as you don't fall in love with top pair. If I'm not already on the button, I like a small raise with AQ to buy the button, if I know I can't thin the field.

You can limp behind with other hands that do play well in multi-way pots, such as small pairs, suited connectors, and suited A's. Don't play your draw aggressively, just check and call if you have the right odds and it isn't going to cost you too much of your stack- you'll often get paid off even when it's obvious that you hit, so no need to semi-bluff for deception.

If you've got an especially strong draw, like an open-ended straight flush draw or top pair and a flush draw, you can make a small, pot-building bet. It's especially nice to do this when you have the nut flush draw, because the donkeys limping any two suited will find themselves drawing virtually dead to a worse flush, and if you both hit, mmmmmm buddy.

Hope this helps.