[Scyld]

As it was already said before in this topic, check auto detailing techinques.
Any high quality automotive quick detailer or spray wax will do the trick. I use Meguiar's products (Last Touch, Ultimate quick wax etc.)

The key thing is to never use dry clothes or even microfiber towels to clean the surface.
Always use damp microfiber towel, spray quick detailer or wax on the towel, not on the speaker. Lightly collect any dust with one side and then use the clean side with slightly more pressure to spread the product evenly. The quality of microfiber may vary! It has to have about 20% polyamide. Never use microfiber for glass cleaning. The cloth from Canton is rather for glass cleaning too, I use it for touch displays )))

You can also use carnauba paste wax to get better gloss and scratch protection. At home it will last years when using cleaning method described here.