I don't go far with underpairs unless its heads up and I have position. Then, I can properly deduce whether Villain is on overs or a bigger pair. Even when you put him on overs, don't ever bet or raise. These bets and raises have no expected value, since if they have overs they'll just fold, but if that have you beat they will raise or call. Usually you should just call the c-bet. Then when he invariably checks the turn to you, either A) Bet and take it down (so he doesn't draw to 6 outs for free), or B) Check behind and call a bluff on the river, this is a play I like to make a lot.

Point is, I am never aggressive postflop with small pp's. I am always passive, unless of course I am bluffing.