If you are going to chase the flush then you have to make sure you can do it as cheaplyas possible. If you are against a solid aggressive player then you want to raise his initial bet. This might not make much sense at first but if used properly the intitial bettor usually just checks the turn waiting to see what you will do. This gives you the opportunity tochase your flush cheaper then if you just called showing your opponent weakness in which case he is usually going to try to bet you off the hand thus making it more expensive in the long run to chase your flush. You will actualy be surprised how many times players will fold to you if you miss and you take the lead in the hand on the turn anyway. To do this I would suggest only doing it with people that actually realize that one pair is the single most losing hand.
Another thing with chasing flush draws and how expensive it is going to be to chase them is the texture of the board. For example if you have a board of say 5h,7h, and 2s then it is much more likely that you will win the hand right then and there if you reraise or at least get to check it down to the river if needed. I in all actuality have put people all in in spots like this for the simple fact that if they are the original bettor out on the flop then they usually don't have a very strong hand. This theory does not always hold true for everone but for most it does for the simple fact thatmost peoplethat flop sets on a horrible board like this are trying to make money with their premium hand, thus they usually just check it giving you the free card you wanted anyway. Aside from a set you usually don't have many worries with a board like this, and you will more often than not pick up the pot with the reraise. Lets assume you do go all in with a board like this and your opponent makes a crazy call with someting like a pair of sevens. Well congratuilations thats what you wanted because you are a statistical favorite in this hand anyway. Lets say you have the ah,10h well you have a 54.1% chance of winning the hand. This might not seem like a huge advantage but remeber casinos offering blackjack are only favored 1% over the bettor and they are making 100's of millions with this advantage.
With boards of a higher texture chasing your flush might not even be worth it. If a board comes up as,qh,kh then you might be up against a made sraight, a double pairing of the a,k or k,q. In either of the aforementioned cases the man is not going to let you chase your flush cheaply. You could try the reraise move here but if you got any kind of retallitory raising back then I say you are better off just letting your monster draw go and living to fight another day.
Everything I have just said is of course situational and depends on how well you know your opponents betting patterns and what not. Aside from my little disclaimer their I hope this has helped a little bit.