The big DR is a good, solid bike. It's easily maintained, just keep fresh oil and a clean air filter in it. New? Used?
If it's used and seen alot of dirt in it's past life, check the brake pads, wheel bearings, etc. for excessive wear. Same goes for the chain/sprockets.
If it's new, the chain/sprockets are pretty low quality, and the best maintenance is only going to go so far. Keep it adjusted and well lubed. Use chainlube or chain wax, whatever you like better. Just use it. Wax tends to attract less dirt when the bike is used on pavement, and keeping the chain clean is important too. If you ride it in the dirt alot, the chain will get dirty and you will need to clean it more often. And lube it more..... just basic common sense will tell you what to do. It's just a chain, not rokit surgery.