Here's another "simple but effective" base.  I did this base a while ago, early on when I first started making dioramas, but never found anything to put on it.  So, I figured I might as well bash something up right quick and throw them on.  This diorama is a very simple three-step base.  You first start out with the wallpieces you want to use.  In this case, I used styrofoam, cut it to shape, added cracks, bullet holes and whatnot, then glued them together and then onto the wood base.  Next step is to mix up some plaster, seal off the wood base with tape so it doesn't stick to the wood parts you don't want it to, then it's time to lay down the groundwork.  The ground areas are lightly laid plaster with sand, gravel and dust added.  When the groundwork is dried, the next thing to do is cover the styrofoam wall with plaster so it strengthens the wall and at the same time, add cool wall-like texture to it.
 

Once everything is dry, it's time to lay down the base paint - matte black.  I use Krylon's "chalkboard" black spray paint.  It's enamel paint and works rather well.  It takes a little while to dry but it looks great when used as a base coat.  One note you might want to consider is that enamel paint will EAT right through regular styrofoam, but in the instant case, we covered the styrofoam with plaster so it's okay.  Once the base coat is dry, it's time to dry-brush everything with a darker colored tan-like color.  Something not too dark, but also not too light - something like a light colored coffee tone.  Because the hightlights and last coat will be sand-colored, your first drybrush coat must be darker than whatever it is you want the final color to be.  The last and final coat is the sand color.  Just use it sparsely because your last coat is just there as a "highlight."  If you're going to spackle up the whole thing with one color, it will look very flat.  So, in all, there is the black base coat, a darker tone of the final color, and then the final color.  Three paint layers make most dioramas stand out.  This diorama is a very early piece I did when I first started getting into dios.  I have, of course, progressed quite a bit since then so check back soon for some better tips and techniques.  This one is for the beginners and experimenters.  We'll get into a more intermediate one soon so do check back.