Clutch tuning can get a bit involved, so it takes some patience and some notes in your race notebook(everyone has one, right?).
Things you can tune are, spring tension, shoe weight, shoe material, flywheel weight, air gap and shoe shape.
The two most common methods are springs and shoe materials though.
I like to run .9 springs and white shoes(teflon) on low traction and 1.0 or 1.1 and black(carbon) on high bite(depending on the engine). I use aluminum and 1.0 only on medium traction tracks, but the aluminum means more maintenance, since the flash formed from wear can make them hang up.
The aluminum shoes have less slip and a firmer initial "hit", so they are best with engines that give a lot of bottom end on fairly smooth tracks. The white shoes give the least grip and offer some slip to help avoid breaking the tires loose as much and also are better in the rough, making the car feel easier to drive under power thru bumps.
You can also fine tune all these set ups by trimming material from the shoes for a later engagement. So if a 1.1 still engages too early and bogs your engine you can trim the shoes to lighten them.
Hope this helps, ....Jim