Bluffing is just believable story-telling, and only works when the person believes, really believes, you have the goods. I make a point of doing at least one bluff in a tournament, without risking my stack.

Sometimes, however, you are pushing one kind of semi-bluff (say a str8 draw) and you are pretty sure that your opponent has two pair going in, and the river delivers on the runner runner flush. This is the perfect place for a flush bluff because people are always willing to cast you in the role of a flush chaser. The bet has to be big and at the upper limit of what the potential caller can afford. All in sends some wrong signals IMHO, to a good player. A good player will fold if he sincerely believes you are trying to extract more of his stack, as opposed to trying to blow him out of the pot.