This tip comes from my model railroading days.

Paint bases, scenry etc with the colour of your ground or earth, then apply flock and other scenic materials on top.

If bits of paint show through it will look like patches of bare ground and actually help to give a more realistic and natural effect.

Kitty litter (unused) and aquarium gravel can both offer good rocks and boulders. A single bag of either will last a long time.

As for clumps of grass I have hear good things about "static grass" but have not tried it myself.

Thanks for the info, Dave.  I plan on trying some static grass this weekend.  I'll let you know how it turns out.