That all depends on the type of finish. If it's mirror polished, you can easily do it yourself. If they're deep, I start with a metal polishing abrasive paper, sanding the entire blade (take the blade off the knife if possible). I start with 320 or 400 grit and move to 600 grit. I have access to 1000 grit as well, but have found it to be pointless past 600. After sanding, I take either simichrome polish or similar (I prefer Nambe's metal polish, but simichrome works well, too), and hand polish once. Then I reapply and hit it with my Dremel on a low setting (use the felt polishing wheels). Finally, a third application an a polish with the high setting. Be careful not to slip as the Dremel may add another scratch.
Anyway, it works for me, and I can't even tell the blade had been scratched. If the blade has any other kind of finish (except hand-rubbed satin) you'll need to have a pro do it.
For had-rubbed satin, just start with a coarse grit paper and work down until you hit the same grit as was originally used. This may look slightly more crude, but with practice I've had good results.
I'm not a professional knife-maker, and they can do it a lot better I'm sure, but on a limited budget with limited skill I've found the above methods to work. Just please remember to disclose any kind of work you do to the knife if you plan to sell/trade it later on.
Good luck,
-Al-
