When you know you've got a really bad cold front coming in, many folks recommend blocking off and insulating your crawlspace vents for those couple days. This can be done by taking a small trash bag, stuffing it with insulation, then placing it within the vented area. You never want to push unprotected insulation to the vent as any direct moisture contact from the outside will ruin its insulating properties. Just remember to remove the block off once the cold front has passed. To ease your mind a bit on any moisture building up during the time that your vents are closed off, it is important to make sure that your dirt floor is properly covered with a good vapor barrier. Each sheet of the barrier should be no less than 6 mil in thickness, and overlap a minimum of 12 inches for maximum protection. The natural water from within the dirt is one of your biggest culprits for introducing moisture into the crawlspace, so a good plastic barrier will reduce crawlspace humidity substantially. You can also add a dehumidifier to the area, running a hose from the water collection of the dehumidifier to your sump pump, allowing for the ultimate in maintenance free operation.