Being in sunny South Africa, the sun is probably the main thing that will put a piano out of tune.

Avoid placing your piano in direct sunlight, as it heats up the piano and it not only affects the tuning but causes fading both of the polish and the keys. "Most piano manufacturers recommend against the use of furniture polish. The best way to clean dust and finger marks off the piano, they say, is with a soft, clean, lintless cloth (such as cheese cloth) slightly dampened with water and wrung out. Fold the cloth into a pad and rub in the direction of the grain of the wood, using long straight strokes. Then repeat with a dry cloth pad to remove any remaining water droplets. If you insist on using furniture polish, make sure it contains no silicone." (From The Piano Book by Larry Fine).

The problem with silcone sprays is if it gets near the tuning pins, they will become loose as a result and the piano won't stay in tune. Inside the piano; It is best to have a piano technician, like myself to help guide you on light cleaning procedures, as any knock of an action part in your instrument may break something and become an added expense.

Too Hot, cold, dry or wet.   For a piano, any of these conditions when extreme and consistent will end up causing damage. Pianos at the coast usually have their fair share of rust and/or mildew. Inland then the dryness of the air causes problems, drying out the wood and causing splits to occur. Ideally, given the circumstances of where you live and available resources then try and avoid sudden changes of condition. Where possible have a Dampp-Chaser  fitted, the long term results outweigh initial cost.