Let me replay a situation which (I'm sure) is familiar to you all. You have J-10. The flop comes K-Q-5 and there are three of you in the pot. You have a nut straight draw and position on the other two players.
The first player bets, the second player calls and so do you. You notice that the flop contains a king and queen, so it is likely that the first player has a high pair as he is betting it out of position. I usually give him creditor for a pair of kings at this point. As such, there is almost no chance of a semi-bluff raise working. You are afraid that raising for a free card in this situation may get you into trouble with the bettor, so you don't choose this option either. Calling is the best option and is mathematically correct, since you are just over 2-1 to make your straight draw and getting 5-1 from the pot.
The turn is a 7. It seems strange to many players, but you should strongly consider folding on the turn unless (of course) you get a free card. If the first player bets again and the second player now folds, you are not getting the right price for your hand to call. I point this out because most players believe that this is an automatic call on the turn, simple because their draw is strong and they have a lock if they hit. But you are losing money calling in this situation.
If the situation above arose in a 10-20 game, there would be $30 in the pot before the flop. On the flop, another $30 went in. This means that the pot is now $60. If the first player bets the turn and the second player folds, you are calling $20 to win $80. In other words, 4-1 for your money. You are 4-1 against winning the pot. Now subtract the rake out of that pot and you are getting worse than 4-1 for your money when you're 4-1 against. Why play a negative expectation situation when there are plenty of positive expectation opportunities awaiting?
If the second player calls, you are technically getting sufficient odds to call. If you take the rake into account, you are just slightly better than even money in the long run. However, I generally prefer to invest my money on situations where I will be paid off handsomely on a draw, rather than barely 'break even.' On rare occasions (but it does happen) one of your opponents may also have J-10 and you end up splitting the pot on the river. This reduces (to a small degree) the positive expectation of your call on the turn. Why play a marginal situation when there are plenty of other good 'positive' situations awaiting?
Also consider that, in the above example, the turn was 7. Hold'em is a game of many hands and the turn can occasionally gives your opponent a backdoor flush draw or full-house draw, which impacts on your drawing odds. Sometimes, the turn may give your opponent a boat, in which case, you are drawing completely dead. In the long run, this factor also impacts your long term expectation of winning the pot on the river. This coupled with the rake should be enough to turn a very marginal call into a fold. I know it's hard when your draw looks good, but that's the discipline required.
Keep in mind that the above example involves a short handed, small pot. If it is a multi-way (larger) pot, it is usually very profitable to call on the turn. Also, the situation would be different if you have overcards with your draw. This is because you have enough extra outs to justify a call. That is, J-10 on a flop of K-Q-x is a problem on turn in a small, short handed pot. If you held K-Q and the board was J-10-5-3, it may be worth calling on the turn as a king or queen could win you the pot. A similar situation occurs when you have a hand like Ad-10d and the board is Jd-5d-7s-8c. Here, a diamond will win you the pot, but an ace or 9 on the river should also be enough. One final note. I will automatically fold if the turn card gives someone a potential flush. Even if the action on the turn would suggest no one has flush, you may be drawing against a small flush (which an opponent is afraid of betting too aggressively on the turn). At best, no one has a flush now, but one has a flush draw on the turn, which means that you have six outs instead of eight. These margins make a difference.
So keep it tight on the turn when you are on a draw and the pot is small and shorthanded.