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Check the rules for your front splitter protrusion allowance and follow it. For me it's 4" in front of the bumper (using a plum to mark the ground plane from the leading edge of the bumper) and I follow 4" ahead of the bumper all the way around. Having the splitter farther in front of the car and more importantly ahead of the bumper, forces more air to go around or over the car instead of under. The biggest mistake people make on the front splitter is making it too small. The second biggest mistake people make is not treating the splitter like a disposable part, (they will be if you're driving hard enough and the splitter's installed at the right height) so take that into account when you make yours. A good splitter can generate nearly 200 lbs of down force, so if you can stand on it, you've built it strong enough.
Side skirts should come as close to the ground as possible and on bugs should be somewhat triangular. The front of the running board/side skirt should go down to the ground next to the body and the trailing edge of the front fender and make a wedge to the rear so that it goes down to the ground at the outside leading edge of the rear fender. That will keep air from piling up in front of the rear wheel. Check out the side skirts on the Jeffery's J/sports racer for a great example.
A rear diffuser is great if you run a tight suspension setup (read: heavy springs) because the diffuser strakes need to be close to the ground. Also, the diffuser should not be much greater than 7* angle from the ground plane or you get flow separation. Ideally, you want to start the diffuser as far forward as possible to generate the maximum amount of downforce but again, there are usually rules stipulating where the diffuser can start (typically rear wheel centerline). A good diffuser setup can generate many hundreds of lbs. of downforce depending on it's size and efficiency.
Once you have all the aero in place and tuned you can nearly double your spring rate from your non-aero enhanced setup. It should come as no surprise that some mod class cars double their weight in aero at 100mph. A lot of my knowledge comes from mod cars and world challenge race teams for both low speed, high downforce and high speed, high down force.
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