hey sn00x you have several choices that I can think of:
1. acrylic modelling paste--kind of pricey but holds it shape beautifully. cleans up easily
2. Celluclay or Scuplyamold--the standard for groundwork. easy to work with and clean up
3. plaster of Paris--the cheapest of the lot. not a lot of working time though.
4. real dirt mixed with white glue. cant get any more realistic than that! use "clean" dirt though.
whatever you decide to use,you will have to apply a layer of gloss gel or Future over your groundwork to ensure it looks "wet". experiment before settling on a technique and good luck!