I lately I have started to think about the issue with draws, mostly straight and flush draws. Under what circumstances would you raise a draw?

-Tournment / Ring game
-Only two players left / many players
-The certainty that you will get the nut draw (Ace-flush for instance)
-You have some real hand at the same time (top pair)/ or not
-Your position

One problem with draws as I see it is that if you don't bet, you may not get anything from the draw since it maybe won't hit.

Any ideas or hints?