Pistons in the block? Or Pistons removed?
If removed, a drill and wire wheel works good. If in the block, and don't want to get carbon everywhere, carb cleaner or gasoline and a small wire brush. I wouldn't recommend steel wool cause little steel bits will tear off and get everywhere.
When reusing pistons for a build, I always used a worn out ring with one end Ground to a point to clean the ring lands....and pistons are pretty tough, small scratches won't hurt them.....Now that don't mean gouges on the skirt are okay, but small scratches on the dome, or ring lands won't hurt anything.
Hope that helps.