[Gonso]

Here's an easy river situation to use as an example:

You get dealt T [img]/images/graemlins/heart.gif[/img]7 [img]/images/graemlins/heart.gif[/img], and raise from the cutoff, and the BB calls. Let's say he's a pretty ABC player.

Flop comes 7 [img]/images/graemlins/spade.gif[/img]Q [img]/images/graemlins/spade.gif[/img]3 [img]/images/graemlins/diamond.gif[/img]. He checks and you bet, then he calls. The Q [img]/images/graemlins/club.gif[/img] comes on the turn, he checks and you decide to check behind. A 9[img]/images/graemlins/club.gif[/img] comes on the river and he checks to you...

If you're going to bet here for value, you have to consider what plausible hands he can actually call with that you can beat. I'd say there really isn't much of anything after the flop check-call, maybe a pair of 6s or ace-high occasionally. So if he calls, your beat and your bet just cost more money. If he doesn't have you beat, he's folding and your bet was worthless anyway. And then, what if he reraises?

Another reason to bet is to fold a better hand, which is another subject on it's own (which would be difficult in this case also).