This article is from the Pinball FAQ, by Andy Oakland sao@REMOVETOREPLY.mit.edu with numerous contributions by others.
If this is a machine you've just bought, by all means clean out all the
insides, carefully. Don't throw out any stray screws, small springs, or
other objects...They might be useful! Watch out for the various service
instruction sheets stapled around the insides. If you find any mouse
droppings, check carefully for wires and cables gnawed thru so neatly
that you can't see the gaps!
Intermittently flickering bulbs may be helped by bending the socket
slightly out of round with needlenose pliers (with the bulb out!) to make
the grounded shell fit tighter. A bit of burnishing to remove corrosion
can also help.
You can use a business card to clean switch contacts. Slip it between
the contacts, press them together, and saw gently back and forth. If
necessary, use Freon, rubbing alcohol or some such solvent on the card to
soften the crud, and use a dry card again afterwards. High-current contacts,
such as the ones on flipper buttons, may require harsher measures. Look
for a "contact burnisher" at your electronics shop or hardware store.
Never use these on the gold-plated low-current contacts, though, as
they'll destroy the plating and lead to corrosion.
If some switches aren't firing, or are firing sporadically, check the
spacing between the contacts ("Dwell and Gap"). Bally recommends
1/16th of an inch. You can adjust the spacing by bending the stiff blade
that's between the two conductive ones.