In a previous post I wrote about taking care of my hands. I would like to write briefly about a few things I've learned about slapping with respect to caring for your hands.
There seems to be a myth out there that you really have to pull hard on your strings to get a good sound. That's not the case: yes you need to pull the string, no you don't need a lot of distance from the fingerboard to get the desired slap. Instead, only use the tips of your fingers and pull enough just to get the string to rebound off the fingerboard. In terms of caring for your hands, your fingertips are already "padded" in the sense that the skin there is stronger than on the rest of your fingers. As a result, using your fingertips avoids injuring the fleshy part of your fingers on the insides of your knuckles. (I've seen some people literally curl their fingers around the string and get stress cuts all the way down their fingers.) By using your fingertips, you also obtain a smooth motion in which the hand pulls away and the string naturally rolls off the pads on your fingertips, slapping against the fingerboard.
Yes you will get blisters doing this. I think that’s just the nature of slapping on an upright bass. The trick though is trying to minimize any serious damage while at the same using a part of your hand that is relatively quick to callus.
The other advantage of using your fingertips and not hitting too hard is that you are less likely to smack your fingers into the fingerboard. Some players I know get these nasty breaks in their skin where it looks like their fingernails are separating from their fingers. We've all heard the stories about some stodgy teacher wrapping her students’ fingertips with a ruler. That's exactly what you're doing each time your fingertips ram into the fingerboard as a result from an overzealous and slightly miscalculated slap.
The basic lesson on caring for your hands while slapping: don't have to slap hard and you don't need to use a lot of finger on the strings. Hope this helps.