I don't have kids yet, but when I do, I'll definitely spank. My husband and I have already come up with a few guidelines:

-only for predetermined specific behaviors (outright defiance, physical violence, for example)
-always in the same place (for example, our bedroom)
-ONLY with a paddle made for that purpose (friends of ours actually had their children decorate their paddle
-first, we have to give ourselves a 5- or 10- minute "time out" (or however long is necessary) so that we are not spanking in anger.
-a limit of 3 or 5 swats
-thorough discussion of the behaviors that provoked the spanking

But once again, we don't have kids yet, so this is all theory at the moment