I agree with what's been said about spanking as punishment, but my master sometimes uses it as a light punishment and it works for me. I don't enjoy the spanking if I know he's not doing it for his pleasure. That said, it's definately not the choice punishment nor the most effective one. Come to think of it, it's really the expression of dissaproval more than the smacks themselves that work.

Also, with areas to aim for, knowing the sub is important. I bruise really easily, so if he wants to avoid bruising he has to stick to the cushy part of my ass. For others it takes a good wallop to produce a bruise.