Here are some tips for kalimba and mbira maintenance:
Do not leave the instrument in direct sunlight, rain or wetness.
You can regularly put a little bit of vaseline (petroleum) jelly on the bottom tips of the keys to prevent rust and it will help your fingers play smoothly on the keys.
Handle the instrument carefully, dropping or bumping the instrument may cause the metal keys to shift and get out of tune.
If your instrument does bump and the keys touch each other, you can separate them carefully by putting a flat screwdriver on the side where the keys are touching and with a small hammer gently tap the back of the screwdriver so the key will move back in place.
It is advisable you check the tuning of your instruments if it has dropped / bumped or as a routine check.