This is a good subject. I used to trade futures (very similar to playing poker) and managing this type of question was critical to success.

In my experience the whole thing comes down to balance and self observation. You know when you sit down at the table and you are feeling just "right" that you will win in an almost effortless way. The opposite is true too, you sit at the table and just don't feel right. Nothing comes off and you get frustrated.

I think managing your internal state is the key. We used to do this by going through a checklist before we started trading and if the answer to any question on the checklist was a no then we wouldn't trade. A very interesting book on this is "The Inner Game of Tennis" in which the author explores the relationship between the conscious and sub-conscious mind. This is what I mean by balance. It has quite a Zen feel to it but that's the way it is. Being in the moment is key and a way of doing that is using breathing as an anchor. This way you can eliminate the mental storm that sometimes takes you over.

Anyway, my 2 cents worth.....