A bowl of boiling water in a hot oven for about 30 mins (door closed), turn off the oven and let it cool.
Will steam most of the crap off, then some hot soapy water and a soft scourer, elbow grease and some time.

Easy as, and no chemicals, works a treat. I do it every month and it means there isn't anything too crusty and caked.

Also, wiping out spills etc as the happen will stop you getting a rally manky burnt oven too.

HTH