Looking after a watch properly can extend its life by many years. There are a number of easy ways that you can do this.
Treat your watch according to its water resistance rating. Every watch has a water resistance rating. For a breakdown of what each one means click here.
Do not expose your watch to rapid temperature change. It is best not to wear your watch in a sauna or in a cold plunge pool. This is because rapid temperature change causes materials to contract, if the seal (that makes the watch water resistant) changes size and/or shape it could mean that your watch is no longer water resistant.
If the air under the glass rapidly cools it may mean that the water in the air will condense. In most cases this goes away without any intervention, but water can get into the workings of the watch. The best course of action to take if this happens is to have the case opened and left to dry as soon as possible.
Once the case has been opened the water resistance rating can not be guaranteed until the watch has been tested in a pressure tank. If you frequently use your watch in water it is best to send it back to the manufacturer as local retailers are unlikely to have the equipment necessary to do this.
Do not knock your watch. Undue shocks can damage the internal workings or scratch the glass. It is generally inadvisable to wear a watch during sports, unless it is specifically designed for them. This is especially important if you have a watch which has moving parts. Mechanical/automatic watches and quartz watches contain moving parts. A digital watch has no moving parts so an impact should only damage the glass. A breakdown of the different qualities of watch glasses can be found here.
Avoid wearing perfume or moisturisers in the same place that you wear your watch. Watch bracelets, straps and the internal workings can be susceptible to corrosion from cosmetics.
Check straps and bracelets regularly. Ensure that they are not loose or too worn. If necessary get the clasp tightened or obtain a new strap, both of these tasks can be done by your local jeweller. Keep hold of any additional links that you have for the watch, these will come in handy if one breaks.
Keep your watch clean. If it has a water resistance rating of 50m you can clean your watch with luke warm water and a very small amount of soap. A tooth brush or similar object can be used to easily loosen dirt. Make sure that you remove all soap and dry it thoroughly afterwards. If the water has a rating of less than 50m the best thing to use is a soft cloth and a dry toothbrush to clean between the links.
Change the battery when necessary. If a battery powered watch has stopped or is losing time it probably needs a new battery. Even if you do not intend to wear the watch for a while it is better to keep it running as the mechanisms inside may seize up if they are not used. It is advisable to get the battery changed by a jeweller as there are a number of small parts which can easily be lost. Remember that water resistance cannot be guaranteed unless it is tested. To find out more about different kinds of power for wrist watches click here.
Get your watch serviced. If you have an automatic, divers or a particularly expensive watch it is recommended to have it serviced every 2-3 years. Otherwise the cost of servicing is likely to outweigh the cost of replacement. If the watch is under guarantee ensure that the service centre used is one recognised by the manufacturer so that the warranty is not invalidated