After every use

Check your clothes' pockets for coins, keys and tissues as you load the washer, because having them bouncing around in the wash is bad for the machine.
Place small items and delicates in a laundry bag to protect them from getting caught in seals and joins, or otherwise damaged.
Double check for any laundry left behind after you've taken the washing out. Damp laundry left in the machine can start to smell, and even foreign objects (such as coins or bobby pins) can leave rust spots on the drum.
Wipe out the inside of the door seal to clean off mould and scrud (the detergent deposits that can built up inside your machine).
Clean the lint filter (if fitted).
Leave the door ajar (if your laundry allows) to extend the life of your door seals.
If your machine has shifted during a cycle and is now off balance and bouncing around, move it back into place or adjust the feet to make it level again.

Monthly

If you only wash in cold water, run a cleaning cycle or run the machine empty on its hottest cycle with a bit of detergent. This will help clear out scrud.
Wipe away mould deposits on door seals and other surfaces using hot soapy water and a soft cloth.
If you notice any scrud build-up inside the drum, remove it with hot soapy water, a sponge and a bit of elbow grease.
Check the wash pump filter (located behind an access panel on the lower front of the unit) for any foreign objects.

Annually

Check if the machine is properly levelled and adjust feet accordingly if necessary. All four legs should be wound down to the floor and the appliance should be levelled with a spirit level.
Scrub detergent out of nooks and crannies with a toothbrush, and don't forget about the detergent dispenser.

Troubleshooting Guide