Make a habit of wiping down the exterior surfaces with a damp sponge every time you wipe down the worktops.
As a pre-emptive strike, if the dish you're cooking is likely to spill, place a baking sheet covered in foil on the floor of the oven. (Make sure the foil doesn't touch any heating elements).
Conventional Ovens
To clean ordinary enamel (smooth to the touch) linings and oven doors, always use a liquid or paste cleaner recommended by the Vitreous Enamel Association.
To make cleaning easier before you begin, place a bowl of water in the oven and heat on a high temperature for 20 minutes. The steam produced will help loosen the dirt and grease. Wipe condensation away before using any cleaning product.
For heavily soiled ovens, use a commercial oven cleaner. Never apply to a hot oven, the electric elements or oven lights. Make sure the room is well ventilated and you are wearing rubber gloves.
If your oven is enamel lined, you can prevent dirt build-up by smearing a thin paste of bicarbonate of soda and water. It dries leaving a protective coating that absorbs greasy soiling and makes it easier to clean next time!
Self-cleaning and continuous-cleaning ovens
Even if you have a self-cleaning or continuous-cleaning oven, don't think you can scrub 'clean oven' off the list of chores completely.
Continuous-cleaning (or catalytic) ovens are designed to work at an optimum temperature of around 200 deg C. So if you tend to cook at low temperatures, turn the empty oven to 220ºC for an hour each week to stop the linings clogging up and keep them working efficiently. Then wipe out the oven with a non-abrasive pad and warm water. But never use detergent or harsh scourers on self-cleaning surfaces (rough to the touch) as it will damage them. The bottom of these ovens is usually enamel so can be wiped clean.
If your oven has a self-cleaning or pyrolitic cleaning system, once you've run the special high-temperature self-cleaning cycle, wipe out the ashy residue with a damp sponge. The frame around the oven opening and the edge of the door outside the seal will also need washing down with hot soapy water.
To clean the racks:
The easiest way to clean these is to put them in the dishwasher or soak in a solution of biological detergent.
Any remaining deposits can be removed with a mild, abrasive cream cleaner or impregnated pad.
Glass oven doors
Glass oven doors should always be cleaned when cool.
Remove cooked-on deposits with a plastic scraper.
For light soiling, wipe over with a solution of washing-up liquid.
For heavier dirt, apply an oven cleaner on a nylon cleaning pad, avoiding the rubber trim.
If the glass is removable, soak in a solution of biological washing powder.