If you play your instrument every day, the bow should be rehaired periodically. If a lot of hairs are lost in playing, rehair the bow. If the hair seems not to be able to grip the string while playing it probably needs a rehair. This happens when the player practices a lot. As it gets older the hair stretches and may become too long to tighten properly. When this happens, attempting to tighten the bow may seriously damage the stick; once again, time to rehair. If no hairs are lost and the player practices very little it is okay to rehair every couple of years. Whatever you do, don’t attempt to rehair a bow yourself — this is a very difficult job even for some repair people.
If you treat your bow with care and clean after every use you can avoid a lot of problems, but it is important to know when you’re in over your head — at that point, call the pros!
A bow stick should be wiped clean after every use. A soft, non-abrasive, clean cloth (lint free) with no oils or chemicals of any kind, should be used. There are special untreated cloths marketed for the cleaning of instruments and bows; there are also many types of cleansers and polishes for stringed instruments that can be used on bows. If the bow is wiped properly after every use, cleansers and polishes are pretty much unnecessary. Never use any kind of commercial cleanser on a bow (or stringed instrument) and keep all chemicals, cleansers etc. away from the hair.