Looking After a Calculator Keypad
Calculators are rarely thrown away because they stop calculating. They are thrown away because a key stops responding.
What kills keys
Dust, crumbs and spilled drinks. Membrane contacts under the keys are what fail, and liquid is the fastest route there.
Cleaning
A soft dry brush around the keys regularly does more than an occasional deep clean. If you use a slightly damp cloth on the surface, keep moisture away from the seams.
After a spill
Power off, remove the battery if there is one, and let it dry completely before testing. Testing early is how a recoverable spill becomes permanent damage.
Use the cover
The sliding cover is not packaging. In a bag it is the only thing between the keypad and everything else in there.