I have asked the same question since my boat lives outside and I use it at least on and off all winter. The tips I have gotten are:
1. Trim the motor down to allow the water to drain (already noted)
2. Cover the Center Console so that it doesn't collect moisture from leaky screw holes or whatever (my boat is older and the console is not 100% water tight)
3. Keep gas in the tank to avoid condensation, but make sure you a) try to always use non-ethanol gas or at least use ethanol treatment, b) run it for at least 15 to 20min at least every couple of weeks, and c) use fuel stabilizer if it's going to sit for any length of time. I use the fuel treatments all winter anyway, since I never know how long it might be till I fish again.
4. Discovered myself: Invest in an onboard charger for your battery or batteries. If they are subjected to cold, when they warm back up they will not have a full charge. A battery left at less than full charge will eventually see the ions fuse permanently to the plates and the total charge capacity will be reduced. If this happens enough times, it basically kills your battery's ability to hold a charge. The onboard charger, which should specify on the box that it is AUTOMATIC (won't overcharge or burn up your battery) keeps the charge topped off every time it warms back up, and keeps your battery from being killed by the cold. It also ensures that when you're starting your cold engine (which may take more cranks than usual) you don't run down your already low battery in the process and leave you hoping you packed your jumper cables. An onboard charger can be purchased for the $20-30 range; after I replaced a battery after only one season, I realized that investment would pay for itself quickly, and it has worked nicely. I leave mine hooked to my boat's starter battery permanently and when I park it, I run an extension cord out from under the garage door, have the plug from the charger sticking out of the battery well and leave it plugged in at all times the boat is not in use.