This article is from the Pinball FAQ, by Andy Oakland sao@REMOVETOREPLY.mit.edu with numerous contributions by others.
To clean drop targets, hold the target up by hand, or remove it entirely,
and use a moist soapy rag or Q-tip. Anything nastier than soap may harm
the paint or plastic. Again, test on an inconspicious place first.
If your drop targets aren't resetting properly, check to see if the
lip the target sits on is rounded off. If so, file the plastic lip (on
the target) flat again or replace the target. Also, check that the reset
solenoid is pulling in all the way so that the targets are coming up to
the correct height.
If the targets don't register when they drop, try cleaning the contacts
as described below.