Here is another in my series analyzing the concepts at the end of No Limit Hold 'em: Theory and Practice by David Sklansky and Ed Miller.
Concept No. 19: Don't call in protected pots without a very good hand.
By "protected pot," Sklansky and Miller mean a pot where bluffing will obviously not work. The most common example is when one player is all-in. Then, there is no point in bluffing because the all-in player will probably win the pot even if you get the other player to fold.
I think this should be good advice in theory, but in practice, it is astonishing how often players will bluff into protected pots. I generally follow S+M's advice and fold my moderate hands in such situations, but, in so doing, I've been bluffed out of many pots. In fact, I've recently decided to relax my calling standards slightly except against thoughtful opponents.
Sklansky and Miller extend this advice to pots that are "protected" by a loose player in the field or by a player who is nearly all-in. Unless you know your opponent is an alert and logical player, I would not suggest taking this advice very seriously. Players still sometimes bluff in these situations. Many will fail to notice that these factors are in play, and many who do notice it will still fail to realize that this means the pot is protected and that they should not try bluffing. This is my personal experience, so take it with a grain of salt.
Surprisingly, the seemingly obvious idea that you should not bluff when a player is all-in (and there is no side-pot) is not supported by game theory. Bill Chen and Jerrod Ankenman show in Chapter 29 of The Mathematics of Poker that it actually can be theoretically advantageous to bluff into protected pots. This is basically because it forces your opponent to play much more loosely. In "theory," all players always know the strategy of their opponents. In practice, of course, this is not the case, and your opponent probably will not expect you to bluff into a protected pot. So, bluffing in these situations is probably never a good idea after all. Actually, if you can get your opponent to suspect that you might bluff into a protected pot, you can forgo ever actually making such bluffs and still gain the benefit of making your opponent play too loosely. I'm not sure how you convince your opponents of this. Maybe pretend to be very drunk?