Do not replace old rusty or pitted pinballs. They add character to the game. The Millennium Falcon is not new and shiny, is it? Your pinballs should not be as well.
If you are going to take pictures of a machine for sale, remove the legs first and place the machine on a cardboard covered driveway first.
Remove the prop bar for the playfield as it is completely, and utterly useless. Just put the playfield all the way up until it rests against the backbox. Trust me, it won't fall. Have you ever seen one come crashing down from lets say, bumping the machine? me neither.
Sandpaper is good for cleaning plastics, but be sure to only use it on the paint side where the plastic will be protected. If you use it on the top, they will get hazy and scratchy, and it will obscure the image.
Powder coating is overrated. Good old rattle can paint from the Dollar store is just as durable. Blast a few coats on wireforms and you are good to go.
A thin coating of motor oil speeds up play on any game. Add some to a rag and apply liberally to the playfield and then wipe it down. No need to wait for it to dry like wax, because it probably never will. If you get it on metal it is an added bonus because it will prevent rust, and your game will have enough character already because of the rusty balls in it. Plus it is not as noticeable as wax on star posts.
A bench grinder works wonders to spruce up a crappy shooter lane. A good rule of thumb is that when you see smoke, you know that it is working.