Preshading

I prime my models in black using a Vallejo primer. AFAIC, this gives me two advantages beside making a base coat color stick better. First, like Jester wrote, it serves as pre-shading. If I want certain areas to look darker, I simply apply the base color less on those areas. Second, it allows me to skip painting rubber wheels in black.

I just recently added pre shading to my list of weathering skills and using the technique that Jester outlined I've had some good success. It definitely works!!

I use one coat for the preshade.  After that, just go slowly with light coats of the primary color until you reach the point that you like.  You do not have to wait for the primary coats to dry, just work around the kit, slowly building the primary color up.  You may also want to thin your primary color a little more than normal also.

This is really a trial and error thing. I use to start out with a model primed in solid black and began applying the top coat over it leaving it thin or not as well covered over the areas where I wanted the pre-shading to show through. Now, I generally prime in a neutral grey which I think gives a more appropriate rendering of the top color. I will airbrush black onto the panel lines and any areas that I want pre-shaded. Next, I will begin with a light coat of the first top color building it up into a more thicker coat in the center of the panels.  The first time I attempted this, I literally obliterated all the pre-shading that I had done by applying too solid of a first coat.