This new section could use more contributions!  For starters, Russ Gilbert
writes (2004.11.05):

The main suggestion I have is to use your A8s. This keeps the keyboard
working.  I didn't have a problem with my 800XLs, but my 1200XLs required
typing the keys a bunch to get them to respond to every keypress.  USE YOUR
A8s.

The problem, I suspect, is oxidation of contacts, in the keyboard, at the
cartridge slot, maybe the SIO port.  Use of a soft eraser on cart edge
connector is one thing I think helps.

I would guess one could take the 1200XL keyboard apart and clean the mylar
traces with ???  90% isopropyl alcohol and a Q-Tip.  I still have my
original 800XL, it has copper switches in the keyboard, no mylar.  I don't
know what my 800s have in the keyboard, but I would guess copper switches.
My original 800XL has all socketed chips also.

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Here is a thread at AtariAge concerning cleaning the heads of Atari floppy
disk drives:
http://www.atariage.com/forums/topic/150716-disk-drive-cleaning/