I can't slap, even if you'd held me at gun point, but I do use my thumb quite a lot nowadays. I started doing it because I saw John Stirratt of Wilco doing that a lot. He puts his little finger under the bridge pickup and I often use the finger rest of my Mustang, like it was made for originally.
One day Timber Tones sent me a lot of big picks, made of wood, stone, bone, leather, the lot. The thick leather ones are strange and nice. They sound like your fingers, but you play different rhythmically.