When should one resign?

So, I played a game recently in which I blundered, and was officially getting crushed.  The position was virtually hopeless for me, and yet I played on, seeing one final "trick."  The trick was rather simple, but my opponent missed it, and I won.  I in fact didn't realize how terrible my position was until I ran the game through an engine after the game and found that the computer thought I was more than -15!  Here is the game.

I thought about resigning several times in this game, but I continued.  I wasn't necessarily expecting the checkmate, but I was hoping my opponent would fall into a trap and give up perpetual check so I could grab a draw.  The question is, when do you resign?  What about the position triggers really makes your decision to throw in the towel?  Do you take your opponent into account?  Any specific examples?

Never give up if you still have a swindle left!

I played this game horribly and was seriously thinking about resigning but I had a feeling that he might forget about the two Queens on the same rank, so hung in there for the cheapo. I must admit, I felt sorry for the guy afterwards and a little embarrassed: