I buy sheets of adhesive neoprene at a local rubber store (google Adhesive Backed Neoprene Sponge Sheet), which I think is the easiest solution. You cut them to size, peel off the backing and stick them on the keyboard like a sticker. The glue they come with is very strong, and they provide extremely good grip on the table - much better than the crappy bumpons or rubber strips that keyboards usually come with.
You can buy the sheets in various thicknesses, and I usually apply multiple layers, with more at the back, to get the exact keyboard angle I like.