There is an answer to this question ,The way to keep your oven permanently clean is to wipe it around with a sponge and soapy water after each use.
Obviously this is not always practical as you probably want to eat the food you have just cooked. Also the oven is hot and you need to leave it for 30 minutes before touching the inside.
It is important to remove any fat or food before it turns solid and become carbonated. Once this process takes place it is then very hard to remove.