You guys realize that Adam asked his question nearly three years ago!
And there've been some other discussions on the same topic. But, as this thread has now come back from the dead, here goes..
Click on the thumbnail below to see what I do with a 20' x 12' tarp while the boat is on the trailer.

(I get the beefiest of the standard tarps and usually they last more than one season. I chose white as the camouflage color in winter and to keep things cool in summer).