Leave the drain plugs out, put a 55-gallon drum under the trailer hitch to keep the boat titled back. If you have hatches, pull the covers and put some of those circular metal house roof-vent deals in them (or the equivalent).
Take a tarp and hang it over the mast and strap town the sides pretty tight.
Basically you want to try to keep the hulls as close to the same temperature as the surrounding air, and moisture free.