Soft Surface: On soft ground, sand, and especially mud consider increasing compression damping front and rear. Sand often requires a bit more rebound damping to rear end kick. Although sand bumps are usually larger, there's more distance between them, giving the shock more time to recover. You may want a little bit stiffer front suspension for sand tracks to help keep the front end up and improve straight-line stability.
In a muddy event, stiffer aftermarket springs front and rear may help, especially if you are heavier than the average rider. Your CRF may be undersprung because of the added weight of the clinging mud. This additional weight may compress the suspension too much and affect traction. Hard Surface: For a fast, hard track with no large jumps, you can probably run the same spring as normal, but run softer damping both ways-compression and rebound.
If you run softer rebound damping, the wheel will follow the rough ground and small bumps much better, and you will hook up better. With a lot of rebound damping, the wheel returns very slowly and doesn't contact the ground quickly enough after each bump. The result is a loss of traction and slower lap times.