The technique that I have used and works well is painting the base a color, like gray, then run a birds feather (you can get one at a craft or art store usually) dipped in paint of a lighter color on the feathered side. Lightly run the feather across the base shaking and moving your hand to simulate cracks and marble color variations. Do the same thing again with a slightly darker color until you get what you like.
Trust me, it sound funny but you will be amazed by it. It looks like marble on your first try! SOOOOOO EASY!!!
Here is a link to a site that has an example using a two tone sponge method for the base.
http://hot-lead.org/advance/texturing_marble.htm