There are ways to increase spring-rate by cutting off a section of spring and replacing it with PVC tubing. Very common technique when prepping street motorcycles for racetrack duty. Not likely to be needed for a hybrid bicycle being ridden mostly on the road.
Use the zip-tie suggestion mentioned earlier to measure how much suspension-travel you are actually using. If you're not bottoming-out the fork, there's no need to increase spring-rate or preload.
Last edited by DannoXYZ; 05-03-10 at 04:44 PM.