I leave my keg sitting next to my keg fridge once empty until I need it. I've found leaving it sealed, under pressure, means that it doesn't get funky in there at all so cleaning is a breeze.
Generally rinse 2-3 times with a half bucket of hot water (throw some sodium percarb in there if there are any spots/marks that don't come off in the first rinse). Then I put a few litres of starsan solution in there, seal it up, and move it around a bit to ensure contact with all surfaces. Pump this through my filter from the beer out using CO2 which sanitises all necessary parts of the filling process. This way, I have an oxygen free, sanitised keg and filter ready to hook up to my fermenter with minimum fuss.