[PygmyHero]

If the pot odds are greater than your odds against making the hand, then calling has a positive expected value.

Here's how you calculate the pot odds: (total money in pot) divided by (the amount of the bet you would have to call).

Odds against making your hand: (number of cards that can hit that help you - your 'outs') divided by the total number of cards left in the deck.

A few important considerations:
1) This calculation becomes a little more complex if there are two cards to come and you are pushing all-in on the flop.
2) Make sure you know what your outs are. For example, your flush card could give someone else a full house.
3) If you are drawing to the nuts, great. If not, you will want a little bit better pot odds to call (i.e. if you are drawing to a flush or straight that could be beaten by a bigger one).
4) Always calling when you have positive expected value may not be correct. For example, if you are in a tournament, and you lose...you're out. In that case it may be better to wait for a more favorable odds situation.
5) You may not want to call (even if the odds are correct)if you're putting a large stack of chips in danger on a speculative draw if your bankroll cannot withstand the fluctuations.
6) You may also want to learn a little bit about the concept of 'implied odds.'

Most of this information can be found in decent introductory poker books. I'm pretty sure Krieger's Hold 'Em Excellence and More Hold 'Em Excellence books cover the above points (except the tournament one - check out Sklansky...Tournament Poker for Advanced Players I think), and many others probably do as well.

Hope that helps!