Taking care of your pinball machine requires that you get to know it well. Observe how it works when it is working well and you will notice if something isn't right. Most newer machine will alert you if a switch isn't working. On the older machines you should be able to notice if something didn't happen when it was supposed to. Make a note of any irregularities, can you repeat the problem? What would cause that to happen? Are all the lights working? Does something happen when the ball hits a switch? Does the flipper respond like it should? If the ball doesn't play correctly, find out why. Something has happened to cause the change. Take care of it when it is a small problem rather than waiting for something major to stop play.
Once you notice something is wrong, get it fixed. It is much easier to fix one problem than it is to fix several. You can do many "minor repairs" yourself. Check under the playfield, you may be able to see whats wrong. Vibration causes wires to break or connections to come loose. Look in the area of the problem and check to see if something is broken. If the repair is beyond you ability or resources, have your local pinball repairman take care of it.
Switches get dirty and fail to make a connection. If you notice that a switch is not registering, make a note of it and its location on the playfield. If its just dirty, you will be able to fix it yourself.
Check out the condition of everything when you are cleaning. You will get a close look at every part of the machine, look for broken or worn parts. Check to see if anything has come loose from vibration or wear and keep everything snug. Replace parts that are broken.