Heres just 2 tips i picked up along the way, Put a fuel stabilizer in the tank and or in your regular tank to keep the fuel fresh, it burns just like normal fuel but stabilizes the fuel for not using it for a period of time. Also go out every once in a while and start it up and let the engine warm up and idle for a while.
The first 2 replies contain some poor advice, IMO.
a) Don't start your bike "every once in a while". If anything you'll just create condensation problems and contaminate the oil.
b) Why empty the radiator? I can see flushing the entire system if the coolant needs to be changed, but just emptying the radiators and refilling with a 75% mixture compromises the overall mixture ratio.
1) Gas stabilizer is good. Put it in a full tank of gas, then run your bike for 10 minutes to warm the oil.
2) Change the oil.
3) Drain the carburetor. Clean the internals with brake cleaner.
4) Clean the airbox and filter. Seal off airbox, or put mothballs in it to keep mice out.
5) Seal the muffler opening (mouse thing).
6) Lube clutch and throttle cable with cable lube.
7) Lube the chain.
8) Spray WD-40 over exposed metal areas to prevent surface rust.
9) Pull the plug, and fog the cylinder, or place a tablespoon of motor oil into it. Crank the kickstart to help distribute the oil on the cylinder walls. Leave the piston at TDC. Put the plug back in.
10) Put bike on a stand to unweight the forks and shock. Bleed your forks.
11) In the spring, drain the tank and refill with fresh premix. Use the old stuff in your leaf blower or something.