Two Small Pair vs. One Higher Pair
This is an area where much money is lost in 7 card stud. It is very frustrating to have been dealt a small pair/small kicker and have bring-in, and then pair that kicker on 4th street for two pair, only to see another player catch a bigger pair on his porch on 5th street.
Correct play is to either raise or fold. Your goal with the raise is to hopefully drive him out completely, putting you on trips, especially when your 2nd pair is pairing your doorcard. This can sometimes be accomplished against good players who are capable of laying down a big pair. However, if your opponent isn't one to fold a big pair, your best option is to fold to his bet. The opponent with the big pair has so many outs to beat your hand that to call his bets is a long term losing play.
If he checks, check along with him, and pray for the miracle full house, unless again, you feel your bet will make him fold.
Another scenario would be that he bet, you raise, he calls, and then checks to you the rest of the way. While betting the very next street, still representing trips might be a good play, you are really in a situation where you will take all the free cards you can get.
Final thought: One of the best ways to avoid this completely is to not play low pairs with low kickers. This should really only occur when you have bring-in with a low pair/low kicker.
Playing low pairs/low kickers is fundamentally a bad play in 7cs, period. In a wild room you might play it for trips on 4th, but your most likely improvement will be 2 small pair, and if one doesn't have the discipline to let go of it, they'll lose alot more long term hanging on to this marginal junk than they'll ever make with 4th street low trips.