Sorry, I only know the experiment-till-it-sounds-good school of compressor theory.
Some tips off the top of my head:
- Hold off adding compression until tracking is complete and you have a rough mix
- Insert volume envelopes to level out the overall volume of the track before adding any kind of compression
- It often helps to precede the multiband with a regular broad band compressor with a gentle ratio (4:1 or less) and slow attack and slow release. Do this if your levels vary a lot.
- The Sonitus multiband is a good one to learn with. It's fairly easy to see what's going on and the controls are sensibly laid out. There are better compressors in the world, but none of them come bundled with SONAR.
- Use slow attack and release times and light compression ratios for low frequencies, faster attack and release times and higher compression ratios for high frequencies.
- Across the spectrum, try to use as little of the effect as possible to get the job done. This tool can quickly turn from helper to destroyer if applied with a too-heavy hand.
- Set your initial threshhold settings to where the compressor is just barely kicking in and adjust downward from there. You want the little bouncing ball to be mostly hovering around the lower part of the "knee".
- Most of the time, the threshold settings of all the bands will ultimately form a line that's gently sloping downward from left to right. If you have one band whose threshold is radically higher or lower than all the others, use the Gain setting to bring it into line
- The default "Knee" setting of 9 may be a bit severe for low frequencies. Sometimes a softer knee is better for lows. But you don't want the knees of each band to be radically different from one another. The exception is if you have to suppress piercing "S"s and "T"s, in which case a hard knee is called for.
Have fun!