Lifted entirely from Derek Schaab's answer. Basically the idea is:
～と: when something is a natural result of some action. Hence, this result cannot be volitional.
～ば: the emphasis here is on the condition (whether it will be met or not is unsure), not the result
～たら: this is the most versatile conditional. It is used when the speaker is more or less sure that the condition will be met, and therefore the emphasis here is on the result. For example, translating "When you're done cooking, call me upstairs" would require ~たら, not ~ば, although there are cases where ~たら can be used instead of ～と or ~ば.
～なら: Probably the most interesting conditional in terms of nuances. Schaab provides a good explanation. It is unique among all conditionals in that it allows you to give a "result" before the condition is met.
Ex. もううちに帰るのなら、この手紙をお母さんに上げてくれませんか。If you're already going home, could you please give this letter to your mom?
Note that the speaker here is requesting an answer BEFORE the speaker goes home.
As we can see, ～なら gives a "while you're at it" or "if you're going to do it anyway" feel. Given that the conditional clause is met, what conclusion or request can you draw from it? That's what ～なら does. This is why, in general, it is not interchangeable with ～と, ～たら or ~ば, all of which require that the condition is met first.
Anyway, let's see Schaab's original answer, lifted verbatim:
