For my Nissan Frontier 4WD, I wash and wax it with NuFinisf-I use two coats applied about two weeks apart. I get under the truck and spray bolt heads, welds, etc with either WD-40 or PB Blaster. I grease the weatherstripping with a high vacuum silicone grease I got from my lab so the doors don't freeze shut. I make sure the 4WD mechanism is working and put a box in my truck with jumpers, tow strap, hook for rear receiver hitch, thermal blanket, candles (for heat) and some energy bars along with spare gloves, stocking hat and socks. I also carry a flashlight with good fresh batteries.
Engine wise, I havea good syn 5w30 oil (Mobil 1 EP) in the engine, have the wiper tank full, make sure the PS fluid is up to the line and have in new engine air filter if needed. I also make sure the cabin air filters are new to get good defrost action. I also keep a collapsible snow shovel in the cab and, on trips, have a bag of cat litter if I think I may need it for traction. This gets me through the mountains of PA during hunting season quite nicely!