I know some people are going to take this as a sarcastic remark, but I mean it in all seriousness. My only real guideline is to c-bet the large majority of the time when I think it's profitable (compared to checking) and checking when I think the bet at that time is -EV. If I think it's close, I tend to go with the more agressive option of betting. How I determine whether or not a bet is profitable is really 2nd nature and would be very hard to explain beyond basic stuff like position, number of players in the pot, pot size, stack sizes, etc. I guess you could say it's just an instinct for me now that I've been putting in so many hands recently.

As far as how much. I'm a big advocate of not leaking information, particularly on early streets, so I -TEND- to keep my c-bets in the area of 3/4 pot as a default. However, that said, there comes times when I think that more (or less) is optimal, and I'll change it up a bit. I try to not be too predictable doing that, however. For example, if there's a KJ9 2 tone flop, I -may- come out and just pot it, whether I'm holding KK or 76o.