Make sure you have shut the power off to the outside unit for the winter, there is a crankcase heater on the compressor, this makes a nice warm winter home for MICE. They will chew wiring, chew off the insulation and leave you with a nice expensive repair bill. Also covering the outside unit, although you may think you're protecting it, will actually prematurely rot out vital parts and condensation will build up because it can't escape. The outside unit has to breath, the best thing to do is cover the top with a board and place something on top to prevent it from blowing off.
Keep the unit looking new every year by waxing it, yes waxing it just like your car!