After your last run for the season, change the oil while the engine is still warm.
Use up most of the gas in the tank on that last run, if not all. Gas will get stale over the winter. Drain whatever gas is left in the carb. Take the spark plug out, squirt just a small amount of motor oil into the cylinder, then screw the plug back in.
Take off the battery (or leave it in the bike) and charge it with an intelligent charger over the winter.
If you have some way of lifting the bike to vertical position (off the sidestand) - even if just a small amount - make sure to rotate both wheels once in a while so the tyres won´t get flat where they touch ground.
If you only have the sidestand, move the bike a meter or so once in a while to prevent the tyres from becoming "un-round" by long contact with the ground.
And finally, give the front forks a small squirt of machine oil where the oil seals are. It will prevent your fork seals from drying out and cracking under the first compression during that first run in springtime.
That should the trick. Good thing winter is far away in most places right now