I got this from somewhere else but this is what I'm going to give a go when I move in to the new house. There were a few people happy with this easy solution.
"This tip is from the Queen of Clean website. Put the oven rack in the bathtub, cover it just enough with hot water and pour a cup of granular dishwasher soap and let it soak for an hour. I left mine in overnight and the grit and grime literally floated off. I wiped them down with an old wash cloth and rinsed them off in clean wather, dried them off and in the oven they went. Simple and effective."
Hope this helps