Here's a link to one method for winterizing your trailer:
http://www.keystoneforums.com/forums...ad.php?t=20540
As for jacking the trailer, putting blocks under it and reducing the tire pressure. That's likely to be more trouble than it's worth, especially if you are planning to replace your tires every 4-5 years (as recommended by most). I'd make sure they are pressurized to the maximum sidewall recommendation and back the trailer onto plywood or 2x10 blocks to keep the tires from sitting in water or slush all winter. Cover them with tire covers if you're storing the trailer outside.