The stainless steel ball guides on most pinball games are grained rather than polished. Over time the pinball will wear away the grain and leave shiny ball tracks (see Image 2). Here's an easy method for returning them to their factory look.
All you need is a couple of 4 inch abrasive balls; one medium (180 grit) and one fine (400 grit). Harbor Freight sells them for $10 and they will last through multiple games (see Image 1 and ignore the blue nylon drill brush, it didn't work as well as the abrasive ball).
The abrasive ball works much better than a sanding wheel or belt polisher since it's easier to keep it parallel to the ball guide.
Use a 2-speed electric drill on low speed or a variable speed drill. Place the drill perpendicular to the grain of the ball guide with the abrasive ball rotating in the same direction as the grain. You could also mount the drill in a vise (or some other fashion) and hold the ball guide against the ball as it rotates.
For light to medium wear the fine grit ball will get the job done. For heavier ball marks use the medium grit first and then move to the fine grit. Image 2 is before re-graining and Image 3 after.