[DavidC]

When you're on the turn, always use "turn to river" odds.

When you're on the flop, use "flop to turn" odds. The exceptions to this are situations where the pot will be big enough on the turn that you're going to be able to see the river card: most commonly this means that you have a flush draw or open straight draw. If that's the case, then use "flop to river odds". When evaluating longshot draws in these situations, like drawing to a gutshot or a lone ace with backdoor flush, you have to consider that sometimes it will be 2 bets cold, and weigh that probability into the future pot odds.

edit: "Make sure you factor in that you will be paying 1.5BB"... crap, thanks Jax!