what's the best way to clean spark plugs?

Chuck them away n buy new ones. Much cheaper and better for your engine, and fuel economy.

suggest replaceing them every 10000 miles or 15000 klms.

I used to glass bead blast the electrode area but you could wire brush them,
but there cheap enought only 4 bucks each so do the chuck thing better off.

I find bead blasting the best method by far.

When I used to ride motorcross bikes, the dirty 2 stroke would foul plugs a lot. I used to just sand them in the gap area, and chip away carbon with a pin from the other areas. Time consuming but I wanted to ride!