I throw some caustic soap (laundry detergent) in water and soak them in it for half an hour or so, the carbon comes off with a nail brush or scotchbrite pretty easy after that.
depending on how mucked up..........a little, you can get a good fiberpad wheel at sears....3 incher. use it on a drill, grinder, etc....works like a champ. really bad, i have used a sandblaster, with starblast (pretty fine material) and shot at a very glancing angle. look just like new, no effect on valve. as mentioned, i have been using torco SSO for 8 yrs now. after 1100 miles last year, no buildup, just wipe them down.