I scrap out the grease in the bottom as needed (about every 3rd cook). Use weed burner on the shelves each cook and occassionally burn and scrap the top inside when needed. I never ever clean the inside with soap and water or high pressure spray (cause then you would have to reseason).
When I had my smaller offset, I put foil down on the bottom of the cooking chamber and pealed it off after things cooled off completely. Doesn't do much good on my big ol hottie.. I'm sure you already know to clean the ashes out ASAP after they have cooled.