04-23-06 [poker]"Protecting" your hand is generally over-rated. The key question is whether they will put more money in the pot later if they don't improve. If they will only put money in the pot if they improve, then you should bet to "protect" sooner than later. If not, then you are better off waiting until you see whether they improve or not and you can put your money in with more equity.
Say you have a pair and you know your opponent is on a flush draw. It's the turn and he checks to you. You can bet here to charge his draw, and he'll call. Your return on that bet is around 80%. On the other hand, if you check behind him and the river blanks, he might bluff at the river, or check-call with a weaker hand. In that case the money you're putting in on the river has a return of 100%.
Now, if you could get both returns that would be even better, but usually they are exclusive, you can either get the 80% or the 100%, and you should take the 100%.