Make sure the battery is good. If it's been 5-7 years since it was replaced, consider a new one.
If it wasn't done in the summer, or you don't remember when it was last done, you might want to replace the anti-freeze. Just a drain and re-fill with 50/50 coolant/water is usually good enough to freshen up the chemicals that will keep the engine cool when running and the block from freezing when it's cold.
Get defrosting windshield washer fluid with antifreeze in it. Prestone and Rain-x make some good stuff, about $2-3/gal. Worth it compared to the blue junk that just ices up your windshield.
Make sure your A/C works good for the defroster when it kicks in. Consider using Rain-x Anti-fog to wipe the interior windows with so they don't fog up. Rain-x also makes emergency wipes for this, too.
Get winter wipers. Regular blades streak and smear when they get snow and ice in the frames. Visibility is critical for winter driving.
Make sure the tires have enough tread. Consider purchasing a set of two or four tire chains or cables in case you need to get out of a ditch or move around in deep snow.
Make sure you have a supply of survival goods in the vehicle, including a flashlight, matches/flares/firestarter, food (hormel chili or beef stew works great if you can't get anything else like MREs that will store well, just be sure to pack a can opener), and maybe a knife, and a small or compact folding shovel. You might also want to buy either a survival/space blanket (foil looking thing about the size of a D-cell battery when rolled up tight), or some other kind of blanket or warm clothing. Worst case, if you get stuck somewhere or can't travel, you can stay with the vehicle for awhile and not get hungry or cold until help survives. If nothing else, you can be warm and have something to eat even if you're just waiting for a tow after the driver gets to everyone else on the way.
Also, you might want to seal up the rear seams under the cargo area floor. The tires kick up salt and snow and eventually rust holes into the floor. Save yourself the hassle later and seal up the seams with silicone, or even use a rubberized undercoating.
