Thanks for posing this very important and intriguing question. Trying to successfuly answer this question has been integral to the success of my poker hobby. Most generally, I fold when I am convinced I'm beat. I often still call if I question my read. At this point I to try to assign a value to the accuracy of my read and decide if the odds being offered are greater or less than the probability I think I'm beat. Generally, I try to enter the phenomenal field of my opponent and view the situation from his perspective. Obviously, I can't know what he holds but I can make conjectures based on my notes (if I have any), previous behavior at the table, and immediate preflop and post flop betting behaviors (time to make bet, size of bet, position, chat, etc). I guess it's more instinctual; I'd have difficulty writing a general principle for folding. I do know in the past I called too much, often against my instincts and almost always paid for it.

JeffreyREBT (Wherein I don't promise to make you rich without trying, or even trying very hard; I do promise to say things that will make you FEEL rich).