Hi Lee: The first thing you really need to do is see if you can locate the source of the moisture in your crawl space. Most often this is due to poor drainage from the foundation, with poorly placed/aimed downspouts the most common culprit. The next most likely cause is ground that does not slope away from the foundation wall. If these are both in tact what is least likely is that moisture is rising up from underground. You can determine the latter by diggind down 3 feet and checking the moisture of the soil down there. If it is moist, this is more likely the problem. If the soil is reasonably dry, the moisture is probably coming from above.
In the less likely event that moisture is rising from below, the only solution I can think of is to dig down far enough so that you can spread 3-4" of sand or 6-8" of 3/4" minus gravel down. This will retard (but not completely eliminate) moisture evaporation and should keep the surface of the crawl space reasonably dry. If you go this route, be sure to mix some boric acid in with the sand or gravel. This will keep the insect population down.
If on the other hand you remove the source of recurring moisture, it should be a simple matter of keeping your foundation vents open (an additional fan or two in the crawl space to expedite the drying process will help) until the surface drys out. Then go ahead and spread the 4 mil (or greater) plastic. down on the crawl space surface.