First thing to do is clean everything real good. get all of the fuel/oil residue off of the planes. If fuel has gotten inside (into the wood) try to get that absorbed. Some use kitty litter, corn starch, talc powder and others that will be mentioned. Tighten/tack down any loose covering and make any needed repairs. The engine should have been burned absolutely dry at the field and just load it up well with the after run oil of your choice. Be sure to put some down the plug hole and turn the engine over a few times slowly before reinstalling the plug. Make sure the crank case has a fair quantity of oil in it and that it has gotten onto all internal surfaces. Take the RX batteries out of the plane. I usually just overnite charge mine once per month during off season and have never had a pack go bad but I know there will be other ideas presented on this subject. I keep my TX's and RX batteries in the indoor closet where thy will not be disturbed except for charging. Store the planes where snow shovels and everything else will not present a danger.
If all this can't be done, just box it all up and send it to me and I'll fly them for you all winter, without charging you either. Nice guy that I am.