I wouldn't recommend this for everybody, but my preference is using a belt grinder/sander (even something like THIS from Harbor Freight) using quality belts and finishing on leather belts loaded with abrasive compound (usually Flexcut or white jewelers rouge). Like I said, I wouldn't recommend this for everybody as you have to be EXTREMELY careful and keep the knife cool or else you can very quickly ruin a knife's temper. However, that being said, I can set a bevel and have a knife with a mirror finish I can shave with in about 5 minutes...
I've seen people use this method, and it's impressive! Very steady hand and great control required! It makes an awesome edge though once you master it!