While today's watches can take quite a beating and still keep ticking, there are a couple of things that you should keep in mind which will increase the life of your watch considerably.  Some of these are obvious, some of these are more obscure, but all of them are simple to do.

1. Keep your watch away from strong electrical currents.  Household appliances are unlikely to negatively effect your wrist watch, but stronger electrical currents or magnetic fields have the potential to mess up your watch's internal workings.
2. Do not let your watch come into contact with chemicals.  This is pretty obvious, but if chemicals get onto or into your watch it will cause corrosion and discoloration.
3. Trying to keep your watch away from sudden shocks.  All watches are shock resistant, but to varying degrees.  Usually your watch manual will specify how much of a shock your watch can handle.  This is particularly important if you work in such a field like construction.
4. Avoid extreme temperatures.  Extreme heat will shorten the battery life of your watch, while extreme cold can actually stop the watch hands from moving.
5. Just because your watch is water resistant doesn't mean that you should shower with it on.  While all water resistant wrist watches can be showered with, it is a better idea not to do so.  While you're washing your watch will getting roughed around a little bit, increasing the likelihood of water or moisture getting underneath of the dial.
6. Cleaning your watch regularly (via instructions provided) only takes a couple of minutes and will improve its lifespan and appearance.
7. Store your watch in soft cloth or something similar while its not on your wrist.  This will keep it free from dust and clean.