There's a world of difference between a good LAG and a bad LAG.
3-betting light against a bad LAG who is going to raise/call/reraise/push all the way any 2 cards at any given time probably isn't the smartest of ideas. Against these players it's better to catch something decent and let them dig their own grave. Remember to bump the strength of your hands up a bit in your mind accordinly! Against a bad LAG for example a holding like TJ which hits top pair on say a 38J board should be considered much stronger than you usually would.
Pay attention to see if the villain gives tells
away with betting.. many of them will bet a standardish amount all the time every time with nothing up to a decentish holding and then suddenly kick it up another notch (bigger bets etc) with an actual good holding. Some do this predictably some do it randomly.
Against a good LAG the advice c9h13no3 gave is good.. you have to be more aggressive back at them than you might be comfortable doing. If you fold every time you miss with no fight or without something strong you are the type of player good LAGs exploit to death.