I just wanted to run some numbers here, as I think I've been playing with some leaks (at the moment, I notice that the tighter I play, the better my results--at least more consistent).

First, if it's HU and you have to call pot, you're getting 2:1. On the flop, there are 7 know cards, so there are 45 left in the deck. The means that the break-even point for calling pot here is 15 outs to win--more than the nut flush, for example, which is at most 9.

If there is one more player in, we have the following: Pot is $10, bet is $10, another player calls $10. Now you're getting 3:1, and that means you need 12 outs to break even--still more than the flush. This is actually a "flush plus gutshot" call.

Now, finally, 3 players in: Here you have a pot of $40, and it costs you $10. So, you're getting 4:1. At this point, it's barely break-even to chase the flush. 9 outs are enough--although you do have to ask yourself what other players have. Ideally, there is a set with some straight draws. If everyone else is chasing non-nut flushes, too many of your outs are gone. If the straight-flush looks plausible, this may not be a very good call, too, as you can get yourself into a horrible mess with nut flush vs. straight flush.

Some follow-up to this in a separate thread.