The door seal holds everything you've ever lost
On a front-loading machine, fold back the rubber gasket around the door. In there you will find water, hair, grit, a child's sock and, if it has been a while, black spotting. Wipe the whole channel out with a cloth and hot soapy water or a mild cleaning spray, working all the way round. Stubborn black marks on rubber can be hard to shift entirely; the aim is to remove what you can and then keep it dry so it doesn't come back.
While you're there, check the glass and the inner lip of the door for a greasy film. Finish by drying the seal properly with a towel, which is the single habit that makes the biggest long-term difference.
The filter nobody knows they have
Most front-loaders have a small access panel at the bottom front hiding the pump filter. Put down an old towel and a shallow dish first, because there will be water. Unscrew the filter slowly, let it drain, then pull it out and rinse away the lint, coins and hair. If your machine has a drain hose tucked behind the panel, use that to empty it first.
If your machine is a top loader or you can't find the panel, check the manual before poking around. Not every model has a user-accessible filter, and forcing one is not worth the flood.
