A good vapor barrier can go a long way toward keeping any moisture in your crawl space from entering your home. Vapor barriers are installed on the floor joists, or ceiling of the crawl space, and work to insulate the house. This traps moisture in the crawl space, where it can be more easily dealt with through ventilation and, if need be, a dehumidifier.
There are many commercial materials available that can be rolled out on the ground in your crawl space to prevent moisture from rising up from the ground. The ground cover can then be covered with gravel to weigh it down and further prevent moisture accumulation and any pooling on the cover.
If standing water is a problem in your crawl space, a dehumidifier is not the most cost-effective way to go about removing the water. A sump pump installation can work to prevent the water from rising into your crawl space, in a similar fashion to how a sump pump will keep a basement from flooding.
If all else fails and a dehumidifier is necessary, you cannot use a household dehumidifier. Crawl space dehumidifiers are designed to operate at temperatures lower than standard units. A crawl space dehumidifier will need a drain line, as you will not want to have to climb down to the unit every few hours to change the condensation catch. Make sure the unit you purchase is rated for your crawl space's square footage.