Offical Apple advice
The offical apple advice page in the Knowledge Base advices the following
Clean your AirPods with charging case regularly with a soft, lint-free cloth. Don’t get moisture in any openings, and don’t use aerosol sprays, solvents, or abrasives.
In my mind, this seems to suggest a microfiber cloth that you might use for removing finger prints on an iPhone screen.
An important item to note is the things not to use. Aerosol, solvents, or abrasives.
iMore also suggests that a microfiber cloth to help keep your AirPod’s case clean. However, they also add some further suggestions which include an electronics cleaning product. I’d urge warning here as if it does cause harm to your AirPods, you’re probably out of warranty.
An Anti-Static Brush
WikiHow adds some advice of using an anti static brush on the AirPod grills to keep them clean. This is an important touch as the grills can pick up wax and dirt but a normal brush might lead to issues.
Your own advice?
Any other tips on how to keep AirPods clean? Leave them in the comments,.