I use my coffee and spice grinders a lot, and I find that they are almost impossible to keep clean. When you're grinding just one thing, like coffee, this is not too big a deal. An occasional swipe with a paper towel is fine.
But cleaning a spice grinder is another story. Spices have oils that are released by toasting and grinding, and these can build up on the grinder, muddying tastes. This oil and the dust that begins to coat the blades and inside of my grinder are hard to remove. The grinder can't be washed, so I would clean it out painstakingly with a damp towel.
Then I learned a quick trick for blotting up those oils and spice fragments. Put about 1/4 cup of rice in your grinder and grind for about a minute, or until it's pulverized into bits. The oils cling to the rice, leaving behind just a thin dust of rice flour which is easily wiped out. It won't return your grinder to a pristine state, but it's a good method for weekly or monthly cleaning and maintenance.
Anyone else have a good way to clean a coffee or spice grinder?