I'm partial to the old tie him to a chair and dope him up with morphene / heroin and then take a hammer to his hands until they're dust. Drop the hammer and come back in 8 hours. Dope him up again and take the hammer to his feet. Just keep doing these 8 hour shifts until he dies.

Or... you could try removing 1 inch x 1 inch strips of skin with a straight razor. Dump some salt in and take another strip from elsewhere.

Nice and fucking slow ; nice and painful beyond comprehension.