If something has spilled inside your oven, sprinkle salt on the residue while the oven is still warm. (If the spill is dry, lightly wet the residue first.) When the oven cools, use a spatula to scrape up the spill, and then wipe remaining residue with a soapy sponge. You can also pour vinegar on the residue and cover it with a damp towel until it can be removed easily.
To minimize future oven cleaning ordeals, line the bottom of your oven with aluminum foil, and replace it when spills occur.