Putting the boat away wrong is how you find a cracked block or a flat, rusted trailer in the spring. A couple of hours in the fall saves a ruined opening weekend.
The engine
Run fuel stabilizer through the system, then fog the cylinders per your motor's manual. Change the engine oil and the lower-unit gear oil — old oil holds water that freezes and rusts. Drain any water from the block on an inboard.
Water systems
Blow out or run non-toxic antifreeze through the livewell, washdown, and freshwater lines so nothing splits in a hard freeze.
Battery and inside
Pull the battery, top it off, and keep it on a tender somewhere it will not freeze. Clean out food and anything damp, prop the cushions up so they breathe, and cover the boat so it sheds water and snow.
The trailer everyone forgets
Repack the wheel bearings, check the tires and the spare, and test the lights. Chock the wheels and take the weight off the tires if you can. A boat that comes out fine on a trailer that does not is still a wrecked morning. Need a shop? Check boat repair near the ramps.