Metal polish and a buffing wheel ala Dremel or bench grinder, or polish and a lot of elbow grease. Wear goggles.
Minor blemishes and small rust spots will buff out with time (along with scratches).
If it's really bad, you can refinish the tool with sandpaper. Start with 400 grit to get rid of the stain, then move to finer grits to put the satin finish back on to your tool. You can keep going if you want, and high polish it. A high polished surface is the least likely to tarnish, but it might be a bit slippery in your hand.