The repair guide from http://homepinballrepair.com/ says:

"Microswitches are enclosed and cannot be cleaned. If they malfunction, they may need to be physically adjusted or replaced."

I found that it is not exactly correct, and if you are a cheap b*stard like me, you might be able to salvage an existing microswitch.

I have an old #black-knight that had a number of microswitches with very high resistance readings, varying from 64 Ohms up to infinity/open when the switch is closed.

Failed (resized).jpg

I simply drilled out the melted plastic holding them together (some can be split apart without doing this),

drill (resized).jpg

then take a picture (for future re-assembly),

inside (resized).jpgDiassembled (resized).jpg

and finally, I used the business card trick to clean the contacts.

clean (resized).jpg

Re-assembly & retest

Fixed (resized).jpg

Nope. I simply pressed the two halves together, and there is enough friction that they do not fall apart. Afterwards, when they are screwed back against the supporting bracket, there is no risk of the two halves splitting since they are under compression from the screws.

Note: some of the newer switches do not have the melted plastic - they are held strictly by friction.

@KenLayton's solution: