As far as punishments go...
Ignoring/isolation kills me. Nothing breaks me down faster than knowing I have disappointed him to that point. Whether it's because he needs space/time to think over my punishment, or whether the ignoring/refusing to speak to me/etc. *is* my punishment...it never fails to make me think long and hard about what I've done, and how much I need to improve.
Humiliation works, too. Nothing hurts more than harsh words, applied well. I'm something of a perfectionist, so failing him is never something I want to do. And when I have, getting my nose rubbed in it always embarasses me to the point that I'll never do *that* (whatever "that" was) again.
Boring/repetitive assignments (lines, mundane chores) are much the same, as I absolutely detest them. They work, but less effectively...I have to really try not to mouth off about how much I *hate* writing lines, as it always seems like such a waste of my time. Never fails to hammer a lesson home, though. Repetion leads to accuracy which leads to understanding which leads to improvement, maybe?
I've always had a pretty clear line in my head, about what is punishment and what isn't. Because I enjoy pain, physical punishments (spanking, etc) usually aren't *punishment* to me, but an extension of playtime. ("Have you been a bad girl today?" *giggles* "Oh yes, Master...I've been soooo bad..") That said, there's a big difference to me between a "playful" spanking and a "punishment" spanking. One doesn't necessarily have to hurt more than the other, but it's the attitude in which it's given that counts.
But then, I guess that's it. Attitude is everything, to me. Knowing I'm disappointed him is what makes a punishment a *punishment*, regardless of how it's delivered. But that's just my 2 cents; feel free to disregard them if you disagree.