I think for myself this works... Anything I can afford to lose, so that I am not speding the rent or grocery money or putting yourself into debt because of your habits.
I also find that people take the fun out of it, if the money means to much. I bet and play poker regualarly, but treat it as entertainment, not as a means to make money. I do pretty well with this strategy and in turn am free to lose tonights monies and feel fine and when I win it is an extra surprise, though I am getting pretty consistent.