It's only way to ensure your machine remains in serviceable condition. Prepping your bike for a long slumber begins with proper cleaning. To avoid the formation of rust you should be sure to remove any mud or dirt from your bike. Apply a good coat of wax to protect the paint as well. Speaking of oil, it's time to change it.
Used oil is full of nasty by-products of combustion like acids and moisture.
The last thing you want is that stuff sitting in your crankcase all winter. Change the filter while you're at it. As I found out with my XL, gas can cause quite a bit of damage when it's left sitting in your tank and carbs. Fill the tank, allowing a little room for the fuel to expand if the temperature rises. Make sure you're cap vent is not clogged as well.