I just finished building the Monagram Classics Midget Racer. I'd like to do the display base to look like a dirt track, and dirty up some of the car with speckled mud or dirt. With it having a driver, I'd like to make it look a little more realistic instead of just coming off the showroom floor.
Any help would be appreciated, thanks.

The base is from an AMT display case.

I usually just get some mud brown paint and do one or more of the following:

Get an old brush, dab it in the paint, and smack it head-on onto the model in strategic locations,

Get an old toothbrush, dab it in the paint, and snap the bristles at the car with your fingertip,

Put the paint in an airbrush, set it for a very coarse spray - heck, put a blob of paint in the nozzle! and splat the paint at the car in short bursts.

I've only built a couple of cars, but I weathered them both that way:
http://www.inpayne.com/models/citroen.html
http://www.inpayne.com/models/tibbetscar1.html

Thats pretty much the way to do it, I even use that technique for blood on all my figures, but you know how much I love gore.

Van

In an article from finescale modeler, a builder used real dirt, finely crushed, mixed with some acrylic clear flat. It looked very convincing.

There's a product called Rustall which comes with three other bottles if you buy the pack - the fourth bottle contains a special silt which is essentially 'scale dirt' and can be used as dirt on models.

http://www.rustall.com/prodinfo.html