Going Straight
1. Put the axle in at a downward (5-10 degrees) angle. This provides two benefits. The first is the 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 doesn't come in 
contact with the body.  This tip is for experts only.  First timers have trouble getting this right.  If you have to email me to ask about it, you shouldn't do it.
    
2. Axles must be in straight front to back. That is square to the body. 

3. After pressing in the axles, test the car for crooked wheels...roll it on the floor. If the wheels are on straight, the car should roll 8-10 feet in a fairly straight line. Should the car turn left or right, you need to tinker with the axle placement without removing them from the car body, until it rolls straight.
 
4. Once you match a wheel and axle together with graphite, keep them together. They wear into each other as a matched set.