Smooth the tread: This is accomplished by placing the wheel in a drill using a wheel mandrel (a tool sold at hobby stores), and then lightly sanding the tread with fine (600+) grit wet sandpaper. To make sure that the tread is evenly sanded, attach the sandpaper to a small flat piece of wood or metal, and then apply the sandpaper to the tread. Continue sanding for about 10 seconds on each wheel, making sure that the sanding time is the same for all wheels.
Debur the wheels: Take off the flashing and seam that was produced when the wheel was molded with a 600 grit or better sandpaper. Inside as well as out. Sand any bumps off the wheel the sandpaper. Metal polish will restore the gloss. Be careful not to break any rules your race has regarding wheel modification. (Narrower wheels have less friction and are better, but often disallowed). Don`t sand too much or you`ll create a flat spot. Sand by hand not in the drill. Heat from the high speed of a drill will damage the plastic wheel.
Have only three wheels touching the track: Raise a front one slightly. There is less friction with 3 wheels rolling than 4. Do not put the axles in at the top of the groove. Put them in at the middle. This lifts the car of the track a bit more and reduces the chance of rubbing on the center strip. Position the axles in at a downward angle (5-10 degrees). The first benefit is only the inside edge of the wheel is in contact with the track. This seems to make the car go straighter with less wobble. The second benefit is that the wheel rides to the outside of the axle and don`t come in contact with the body.