In Middle Limit Holdem, by Jim Brier and Bob Ciaffione, they mention the importance of the turn raise. Against typical players in typical situations (in these typical times), a turn raise is big sign of strength. Most players will wait until the turn to trap you for an extra bet, so have a lot more respect for raises on the turn than you would on the flop. This applies especially to players without a great understanding of strategy. If I had a made hand on the flop and get raised on the turn, I generally need at least 2 pair or better to feel good seeing the river. Deviating from this rule requires that I know my opponent fairly well. Folding for turn raises is the most important place to save bets when you hold less than two pair. However, if you follow this rule too strictly, you could be giving up a lot of bets.

In terms of saving bets on the river... the same general rule applies to an even greater extent. Strong hands will wait until the river to trap you for even more bets, so when you are raised on the end, you need to fold anything less than two pair. If you are overcalling a river bet, you need to understand that it very likely that at least ONE of your two opponents have you beat, and it may not be the original raiser.

Of course, all these folds depend on the texture of the board and the nature of the bettors. But the general rule is that you are beat unless you have a strong reason to suspect otherwise. Facing turn and river raises should force you to seriously re-evaluate your hand. You need a good reason to not fold.