I sprayed mine once with PAM. Bad move. Kinda sticky. I have hears some use a quality high temp oil like canola and rub it down. It'll collect dust though. You can rub it deeper so that's not a problem. I don't do that, I live in a very dry climate and my 640 is kept under a cover under a porch. No rust.
After every cook I clean out the fire pot area with a small vacuum. Every once and a while I'll scrape the bottom with a PLASTIC scraper (Home Depot) in the paint dept and vacuum the ash up. Once or twice a year I scrape the full inside well, give it some Simple green and scrub a bit. I cover the auger area with foil, and ensure the pellet bin and auger area is 100% free of pellets. I wash the outside at the same time with a green style sink sponge and soap.
The Grates I wipe down after cool down (still warm) with a stainless scrubbing pad like steel wool just to get the chunks off. I clean the diffuser plate now and then with a metal scraper, usually after every cook.