How I do it depends on if the dirt is wet or dry. If wet/damp, I put the truck securely on a stand and run some full throttle bursts to spin it off. Do this outside (unless you want your house spattered in mud) and stand to the side.
If the dirt is dry, I'll break up the dirt by squeezing the tires. Then, set the throttle trim so it runs about 200 rpm without a trigger finger, and hold the truck over the stiff bristles of a push broom.