Wet/dry sandpaper. Choose the grit based on desired finish. Anything between 400 - 800 usually leaves a 'satin' finish, and 1000 - 2000+ will begin to produce/restore a mirror polish. Avoid 320 grit or below, as it'll make scratches that are very deep, and will be more difficult to refine/erase. Start at the highest grit possible (approximating the finish of the blade), and step down to lower grit only if it isn't effective, or much too slow. Once above 2000 grit, then the polishing pastes become much more effective. Polishing pastes like Flitz/Simichrome usually won't remove scratches that are very deep at all (it'll make 'em stand out even more, as the surrounding un-scratched finish becomes more polished), so the surface needs to be in pretty good shape beforehand, with deep scratches minimized as much as possible.
Try the polishing pastes first (you might get lucky), and if you find it's not quite working, then go to the sandpaper, and work back up the grit sequence (don't skip grits).
David