Keeping you pinball machine clean is part of ownership just like it is for your car. If you want it to last and work well you have to keep it clean. Part of the fascination with pinball is your connection with a machine, cleaning and maintaining it improves that connection. The pride that you have in keeping your pinball machine in top condition can be just as rewarding as playing the machine.
Starting with the playfield glass, use a little glass cleaner and remove those smudges and marks. The better you can see the better you can play. Be careful not to use too much, you dont want cleaner running under the glass. If you spill something on the glass, wipe it off immediately. If it runs to the bottom of the glass, remove the glass and clean the lock down bar and anything else that got hit. While the glass is off the machine clean the backside, I'm not sure how, but it gets dirty too.
Now for the playfield, first you need to know if you have Mylar (clear plastic) protecting the surface. In most cases the Mylar only protects the high wear areas, so you will be able to find the edge of the plastic. If you do have Mylar on the playfield, you must not use any cleaner that might cause the Mylar to lift up. Some cleaners break down the glue and allow the clear plastic to come up. Thats fine if you want to replace it (a very big job) but thats not what you want when you are cleaning. My favorite cleaner is Novus #2. Its made to clean and remove small scratches from plastic but it will clean just about anything. I also like rubbing alcohol but keep it away from the Mylar! Be careful when you are cleaning, you dont want the rag to get caught on something and break it. Some tight spots will be difficult to get to, it may be worth while to remove some of the objects on the playfield, do your cleaning and put it back together. Clean the playfield anytime you start to see ball tracks.
On the issue of how tight plastics should be attached, my answer is with a light firm pressure. There are two problems to be addressed, warping and breaking. I believe that the pinball designers used plenty of fasteners to help keep the plastic straight (flat). To do that the fasteners would have to be tight. The theory on preventing the plastics from breaking is to allow the plastic to move when hit. To me that does not mean loose but to be firmly attached so that when a ball hits it, energy is absorbed by the plastic squeezing (moving) between post and fastener. If the plastic is too loose, the plastic moves with the ball, stops suddenly at the post and then feels the full impact of the ball. So my position is to tighten the plastic down firmly in both circumstances but not so tight that it couldn't move a little if it had to.
If you tighten a plastic down and the pressure of the tightening causes the plastic to move slightly, I think you have it way too tight. In doing this, the plastic gets smashed and deformed over time.
As for slingshot plastics getting broken, you can place thin washers under the plastic that extend all of the way to the edge. That way the ball hit the washer, not the plastic. The problem is that it looks ugly when the pin is lit up and the washer blocks the light. Recently, somebody is promoting a strong clear plastic washer to replace the metal one. It sounds like the best solution to me but I have not tried them.
If the underside of playfield plastics are getting dirty (turning gray) it may be time for a complete tear down cleaning. If you do this yourself, plan ahead, do small sections at a time, take pictures so you can remember how everything goes back together and give yourself a lot of time. It may take an entire weekend to take everything apart, clean it and reassemble it. While you have the playfield apart, you might want to replace the rubber rings, they are dirty and less responsive that new ones. A new set of rubber rings just for your machine will cost between $10 and $20, highly recommended. If this sounds like too big of a project, find someone in your area to do it for you. Check that the steel balls are smooth and have a nice "see yourself" shine. Balls with any roughness on the surface acts like sandpaper on the playfield and should be replaced with new ones.
Wax the playfield to protect it from wear and to keep the ball rolling smoothly. Johnsons Paste wax works well, check out other products made just for playfields. For home play, waxing once a year is probably enough, it depends on how much use the game gets. Watch for any signs of wear when you do your regular cleanings. You might want to put some wax on heavy wear areas when you clean it. You should definitely wax the playfield whenever you have it torn down for a complete cleaning.
The inside of the cabinet should be vacuumed out every now and then. Unplug the machine and lift the playfield (dont forget to remove the balls) Be careful to pick up the larger pieces of debris by hand and not to get caught in the wires. Metal parts fall from the playfield and accumulate in the cabinet. Some of these can be rough on a vacuum cleaner. Clean the outside of the cabinet with your household furniture cleaner. The area around the flipper buttons can get pretty dirty.
The backbox is cleaned much the same way as the cabinet. Be especially careful with the backglass or translite. A little window cleaner applied to a soft rag or paper towel, gently rubbing on the back side backglass should clean off the dirt that accumulates. Stubborn spots can be lifted with Novus #2 but use it at your own risk, it can clean the ink right off the glass! If your backglass is cracked or lifting, dont clean it without getting more information. Ruin the backglass and your whole machine is ruined. Clean the front of the backglass just like the playfield glass.