I semi-bluff one out of every four or five flush draws with a confident bet that screams "I have top pair." Especially when no one else is betting, or if they're just min betting. It disguises the hand, confuses opponents, and gives you a shot to win money with a hand that might not ever become anything. It also makes the flush pay off bigger if you do hit it, because you can run a sequence like this:

FLOP (flush draw) bet
TURN (flush hits) check/call
RIVER bet/raise

or in late position

FLOP bet
TURN check or call behind
RIVER bet/raise

Basically you rep a made hand, check or call out of apparent fear of the flush, and let them bet into you. Aggressive opponents are always good for at least one bet close to the size of the pot on either the turn or river, which is often more than you'll get out of them if you bet yourself when the obvious flush card hits.