The river..

[KeepBlinkin]

The key-point is this:
If you intend to bet on the river, make sure that there's value in it.

Let's look at a not-entirely-uncommon example:

You're holding 2♦ 2♣, and the board shows
A♣ 10♣ 8♣ 6♥ 6♣

You're heads-up, it's checked to you. Do you bet?

Against somewhat decent competition, the answer is "no". A worse hand will not call here, and a better hand will not fold. Of course, if you're playing truly awful opponents, it's possible that you might get called by something like a pair of aces, because they want to "keep you honest", but don't count on it.*

If you get called, you will probably be beat. When that happens, you will regret betting. If you bet, and your opponent folds - did you really gain anything? No - he most likely had the worse hand anyway, and so if it had gone to showdown, you would have gained the same amount anyway. This is a situation where you, in most cases, will simply not gain anything from betting, but you stand to lose one big bet if you're called.

In conclusion: If you're betting a legitimate hand, ask yourself this: Will a worse hand call? Will a better hand fold? Will you win the majority of times that you're called?