How to clean spark plugs?

On my engine I often have the problem that one or more spark plugs doen't ignite properly. Some people say I should clean the tip with a wire brush while others say that I destroy the coating this way. Furthermore I've also heard that a brass wire brush should damage that coating. So what is the correct?

On the occasions where I have needed to clean fouled spark plugs I've used a bead blasting machine. I've never had issues with doing this and have used this technique in many of the high boost drag engines we've tuned and built. I wouldn't personally be concerned about using a wire brush to achieve the same result though. Once you've cleaned the deposits off the plug I'd also be sure to clean the plug down using brake clean and compressed air.