A propane torch can burn off some oil and soot. I don't like to scrape them with a knife or use a wire brush because I think some metal can be scraped off onto the insulator and help it short. I've used my sandblaster, but use fine grit and no more than 40 lbs pressure, then just blast long enough to get the soot removed. You must get all the grit blown out or it can damage the engine.
If you do a lot of short drives, take a long drive and that may also burn them clean, but as Mitch said, you may just need a hotter plug.