I agree with chattykathy-ovenpride is the easiest, most painfree way of cleaning an oven. It provides you with a big plastice bag that you put all your oven racks in and you pour in the solution and leave it for a few hrs(I always leave mine over night) and it just dissolves any grease, baked on food etc off the racks and you just rinse them off. You can also use the solution on the inside of the cooker( not self-cleaning linings though) to clean that too. Brilliant! I thonk it costs about �3-4.
A very inexpensive one that is often available in the kitchen is bicarbonate of soda. Make a paste and apply to the oven., warm the oven, leave to cool and wipe off the paste
Just make sure you are careful, whatever you use! I used oven pride on my BBQ and got burnt! I wore gloves etc, but managed to get the stuff on my forearms and had quite nasty, sore burns. I have since used an oven cleaning service which cost about �35 and it was incredible...the whole cooker looked showroom new and the guy was really clean and tidy. The service was called Ovenpride but has recently changed its name!