to prevent dampness in a crawlspace you need to prevent the moisture from entering. It enters mainly in 3 ways, ground moisture, water run-off, or condensation of hot humid outside air.
The first step is to poly the dirt floor. The poly barrier traps the moisture in the ground so it can't enter the crawlspace.
As to run-off, be sure the the landsaping around the foundation directs the water away from the house. Also, be sure that the foundation is as waterproof as possible.
As far as condensation, a cool crawlspace is one area where ventilation to the outside is not always agood thing. If you have vents, close them (unless your floor is insulated, you shouldn't have them).