Here's a rule: TPTK, bet at least half the pot, if not the whole thing. If YOU are the one chasing, the chips in the pot and in front of other players (meaning about to go in the pot) have to be more than 4x what it will take to call AND if you hit your flush you better hit with a high hole card (like an Ace, because when you hit it you want to win) AND you don't have a reraiser behind you. If it's a multi-way pot and you're in the middle and there's a perpetual raiser to your left then you probably should fold because if he raises the pot, then what ru gonna do, call again?

Basically if SOMEONE ELSE is drawing to a flush I say they have a 1 in 3 chance of hitting it and bet 1/2 the pot to get them to make a bad call. Guess what? 1 in 3 times they HIT IT. What ru going to do when that happens? Whatever you do, don't pay them off with additional bets or calling their bets. Why would you? If every indication is that they were drawing, AND THE DRAW HITS, then DON'T PAY THEM OFF. Fold. That's how you make a profit on the 2/3 times they don't hit. By not paying them the 1/3 they do.

Good luck.