The first thing to keep in mind if the wooden furniture has to be kept spick and span is to make sure any liquid spilled over it must be removed off it within a few minutes. Even water can stain a wooden desk or a chair because there is always a layer of lacquer on top. On top of this, water may not be very clean and may contain various kinds of chemicals.
You never know what will react when and leave a bad blotch on your furniture; wipe off immediately therefore. If something like tea falls on a stool or a writing desk, then the stain can be cleared off with a mixture of vegetable oil and alcohol. In any case, instant removal of stains is important and usage of an alcohol based cleaning agent is best suited for the purpose. Make sure you always use a soft cloth for scrubbing any hard stains left.
Never keep your wooden furniture exposed to the direct rays of the sun because this leads to the cracking of the lacquer layer and exposure of the wood to the weather. Anyway, humidity will always affect wood if it is porous but if you have teak or any other hardwood furniture, then humidity should not be a headache for you.