Wash out the inside and that means under floor mats. Wash the outside and a coat of wax would be good too. If you have a LQ trailer, make sure you do the RV antifreeze in all the water lines, etc. Pull out any food that may go stale or invite mice inside. Then put that trailer away somewhere 'til the spring. Or if leaving out all winter, placing the trailer (tires) over  some wood or cement(to protect the tires) and invest in some tire covers or put boards over the tires to protect from mother nature's elements. I've heard mixed reviews on placing a tarp over the trailer, traps moisture under tarp, bad for steel trailers, etc. Not to mention wind can create havoc for a tarp.

I'm sure others have more suggestions, just a few that came off the top of my head and some I've been thinking of doing to my trailer real soon.

since this was my first year with my LQ trailer I did the following- ran Rv anit freeze in the water line, cleaned out the LQ- food,personal stuff,blankets, cleaned the floors, vaccumed- in the spring it was pretty much ready to go. I hauled out the mats,cleaned the horse area. I left mine outside all winter and it was fine in the spring. I did not cover it ( I would have preferred to keep it inside, I had a early show in march so we had to dig the snow from around the tires to grease the bearings.) I'd say use some commone sense here-clean things up, do what you can to protect what can freeze and some general maintence.

Clean it, clean it, clean it . . . lube it, lube it

and do it before it gets too cold 'cause then you won't want to do it.