[einbert]

My rule of thumb is this:

Never ever bet the turn with position with two big broadways when you have a gutshot heads up. Check behind and decide what to do on the river, because your opponent has a pair or enough to at least call you the vast majority of the time.

This may be too simplistic, and I'm sure there are exceptions (vs insanely passive opponents and vs maniacs betting is quite possibly better), but for the most part I think this results in the optimal play.