If I'm in late position, I always always bet the flop if it's a flush draw. Not only are you getting money in the pot, but you're increasing your chances everyone will check to you on the turn, thus buying a freecard if you need it. If i'm OOP, I still might raise, something near a third to half of the pot, in order to prevent anyone trying to price me out, and thus folding everyone else as well. Honestly, I think you -have- to bet. If you lose 1 bet 55% of the time, and win 3 bets 45% of the time...well it's math!

On the second question, if someone is drawing, I'm making it expensive if I'm ahead. If your opponent calls you everytime, he's making a big mistake, and thus you're correct in your bet. If you do the same on the turn and they call again, you're making even more. Plus, it's easy to see if the flush hits on the turn, so you'll have no trouble getting out of the hand.