Check-raising is best in early position, either in a large field (where someone is sure to bet) or with one or more known aggressors in late position who can't stand to see a pot checked around even once. If someone else pre-flop raised and you act before them, you're golden; most people will cough up a bet on the flop if they raised before the flop, whether they hit or not.

I usually don't bother check-raising TPTK unless I'm specifically trying to trap someone who is very aggressive and I think buys a lot of pots with marginal hands. I check-raise two pair or trips all the time, and sometimes straights and flushes. It's a good idea to make the raise bigger if the hand is more vulnerable, and if the field is larger. In a pot with four other people and having two pair, I'd check-raise a pretty large amount to discourage callers. If I flopped the nut flush I might just check-raise the min to encourage more action (which you won't get from a good player, but might get from others).

It can be a good move but don't over-rely on it. There's an epidemic of check-raising in a lot of online games already, so often you can get as much or more money in the pot by just betting out your good hands and hoping the bad players call. I try to mix it up between check-raising and betting out with a variety of hands. Frequently it depends on who else is in the hand.