I only have two (and one is really just a "best effort" type deal)

1. Never lie, even under everyday circumstances. If you must lie, have a really good reason for it.
2. Never break a promise under any circumstance.

After watching and loving Lie to Me, I really got interested in the whole radical honesty thing. I don't abide by those "shout everything that pops into your head" rules, but I do make every effort to never tell a lie. After a year or so of it, it's just sort of become a part of me; I literally will not knowingly tell a lie unless it's incredibly important that I do so. This makes for some interesting phrasing acrobatics on my part, but it's satisfying knowing I cannot ever be caught in a lie.

EDIT: I also exclude sarcasm from my "don't tell a lie" rule. That should be obvious.

The second one (the whole "never break a promise" thing) I consider more binding. Since adopting the "ethic" I've only made two promises.

A bit strange, I know, but I don't really have any other ethics or values. Play Halo with me and you'll know this is true.