mari,

Just my 2 cents on this, please feel free to disregard if you disagree. Since I'm a masochist, spanking is rarely a punishment, because I enjoy it too much. Sometimes it is, and the look in his eyes, or his tone, attitude, etc. is what really brings me back in line. And sometimes it is nice to have a physical reminder that will last beyond hearing about his disappointment with me. (I like bruises, for this reason!) I've said it elsewhere before (I think) that for me, the worst "punishments" aren't physical, but small annoying/repetitive tasks or a simple lack of contact. Being ignored is an almost instant attitude adjuster.
And as for the "clean slate" feeling at the end of a punishment...I completely agree. Sometimes if I feel I've done something wrong (even if he doesn't think so) being punished helps me get my head back on straight. I know I like to push buttons/boundaries, to see what I can get away with--and I like it far better when he catches me and brings me back in line that when I *really* get away with something.
But that's just me...like I said, feel free to ignore or disagree with me!