String instruments do not like great changes in temperature. If too dry, they crack. If too damp, they swell (especially the pegs) and are extremely difficult to tune. So, keep them in a climate-controlled environment, if possible. Clean excess rosin off the instrument and strings. Keep the bow well rosined and be sure to loosen the hair when not in use. (And no, the strings aren't made of horse hair - the bow hair used to be that. Strings are made from different metals and I've never had one rust. They usually break long before that if put to good use.)
Offered by Michael.