Sweat, always works a treat.
Not too much, just enough to shine the ball. I always get asked how I manage to keep the balls I use in nets so shiny, you just spend the time that you aren't running up and bowling shining the ball. There isn't really a secret magical way to keep them shined, you just do all the time. I've never tried this, but if the ball is scuffed, perhaps use a little sandpaper to smooth it, then start shining it. I haven't used anything other than sweat and saliva, and they both work a treat.