I would just replace them for my vehicle, but for my yearly lawnmower tune-up, some brakeKleen solvent, gentle cleaning with steel brush, and emery cloth on the electrode until shiny(2 swipes?) has always worked! Cheers!
To properly clean spark plugs your local garage may have a media blasting plug cleaner.. However you can do almost as good a job with a wire brush.. No need for any chemicals etc. The wire brush will get rid of most deposits and leave the plugs in a very usable condition.