To properly corner balance your car, you need to have shocks with adjustable spring perches (i.e. Track Pack Bilsteins, Ohlins, Nitrons, etc.), and a set of weight scales. If you do this often enough, or are a car nut like me... you might buy your own set of scales; otherwise maybe you can borrow some from another nut.
The scales I have are from Longacre Racing; I've also purchased their roll-off levelers... that way I can compensate for my garage being about as level as the Arctic Sea during an episode of "The Deadliest Catch".
To level the scales, I use a contractor's laser level (Stanley makes a nice one for <$100). I set up the laser to be just slightly higher than the roll off levelers (10mm)... then I set each leveler to be an equal distance below the laser line by using the adjustable legs. I also try to adjust the legs so that the weight scales themselves are level on the leveler's platforms.
Now, since the weight scales are level, I can just measure the distance from the laser line to the car's chassis, adding the distance from the laser line to the top of the weight scale (in my case ~10mm).
Next concept... for those of you who might not be familiar with "corner balancing"... think of a four-legged table... if the legs aren't all the same length, the table will wobble. To fix the wobble, you can add a matchbook under one of the shorter legs (since it's harder to shorten a table leg than lengthen one). A car's suspension is similar... if there's a difference in the "height" of the corners, the car will not be as stable as if the heights match. The standard way to determine this is to compare the weight exerted on the left front + right rear wheels with the weight exerted on the right front + left rear wheels. With the scales I have, the "corner balance" is the ratio of the RF + LR weights divided by the total weight of the car. The goal is obviously 50%.
In isolation, tightening a spring (by lowering the upper spring perch) will raise that corner and increase the weight it exerts on the scale... loosening a spring (by raising the upper spring perch) will lower that corner and decrease the weight it exerts on the scale.
So if your corner balance is off, what do you do? If your RF + LR weight is greater than your LF + RR weight, you have the following options:
1. Decrease the spring compression of the RF (move spring perch upward)
2. Decrease the spring compression of the LR (move spring perch upward)
3) Increase the spring compression of the LF (move spring perch downward)
4) Increase the spring compression of the RR (move spring perch downward)
or, some combination of the above...
So what combination should you use?
This is where ride heights come into play. If your ride height is good, and you don't want to change it... doing all four equally should leave your ride height unchanged. Doing 1+2 will probably lower the car at all four corners, doing 3+4 will probably raise the car at all four corners. Doing just 1) will lower the RF and raise the LR, doing just 2) will lower the LR and raise the RF, doing just 3) will raise the LF and lower the RR, and doing just 4) will raise the RR and lower the LF.
Let's say your corner balance is good and you want to change ride heights...
Moving spring perches upward or downward (to lower or raise height, respectively) on adjacent corners (i.e. LR+RR, or LR+LF) has a minimal affect on corner balance while affecting ride height at those corners.
Complicated? Well, a little bit. The best way to approach it is to take notes, make small adjustments, and be patient. See if the adjustment you made affected the ride height and corner balance in the way you expected... if not, figure out what you did wrong.
This is an admittedly terse description of how to corner balance and set ride height... I suppose I wasn't sure how much detail to get into. In any case, feel free to critique, ask questions, etc.
EDIT: Forgot this... very important... always ballast the car before doing any suspension measurement. If you track your car and drive alone, ballast for your weight. If you drive most often with a passenger, ballast for both you and a typical passenger. Also, inflate your tires to their "hot" pressures. You might also consider disconnecting one side of your front roll bar so it doesn't bias the front weights.
EDIT2: This post is more of an explanation of the concept of corner balancing... I'll try to put together a series of pictures documenting how I set up my workspace and proceed to set ride height and corner balance. More to come...