What kind of maintenance should I do for a Yamaha wr426 that's been sitting for over a year?

It really depends... I'd probably start with a carburetor clean. Really no need to get a carburetor kit. Just take out the pilot and main jet and make sure none of the holes where it sucks in air is covered or it will run like crap. Check the front and rear seals if it's leaking oil, it's probably a good idea to change out the seals and oil. Probably stick with 5w front and back. (Check with the manual for rear shock oil) Check the chain. Clean if it's got rust. Check inside the gas tank. If there's been some bad gasoline, they tend to jello faster. If it looks ok, just fill it up and it should be ok. Put a fuel filter on it! (Make sure it only needs gravity to get the fuel flowing.) Lastly maybe a valve adjust. It's really a pain the butt, but if you're considering keeping the bike for a while I would consider it. BE ORGANIZED WHEN YOU DO SO!!! Check for cracked hoses in case you have idling problems and such. Good luck.