I'm building and painting a Tau Riptide Battlesuit for a friend of mine, because it's a totally awesome looking model. Here's the base:
Click to enlarge
I glued the plastic in place, then built up the base with cork from a pin-board sheet. After that, I mixed fine gravel with Liquid Nails and covered the cork for a nice rough texture. Finally, I topped that off with a liberal sprinkling of large talus. Painting this base is going to be half the fun!