I don't ever like to bet someone off with a flush draw, I want them to come along and chase, that is how u build the pot. If u really put them on the draw then over bet, or make the pot size bet, it can get u pot comitted to calling of the rest of your chips if the flush hits. I think the more u bet the more juicy the pot looks for the guy with the draw to call. But if u don not like what I said then just bet more than the pot, this way he gets no odds on his money to call. Say the pot is 300, bet 500, 800 in the pot. He would have to call 500 to win 800, not getting odds to call. But doing this also gives away your hand, your telling him I have a hand now and i do not want u drawing out on me. So he knows if he hits his flush it will probably be good.