Most guitars with single-coil pickups suffer to some degree from hum. Almost all are much noisier than they need to be, because the manufacturers don't bother to shield them well and don't follow simple electronic wiring principles that have been known for decades. This holds true even for many premium guitars costing thousands of dollars. Even some guitars equipped with humbuckers can benefit from good shielding and grounding – guitars that pick up local radio stations are good examples.  
These instructions apply primarily to guitars with regular magnetic pickups. Doing the shielding won't cause any problems with newer "noiseless" pickups like Lace sensors, but neither will you see much reduction in hum (which should be almost nonexistant anyway). You may, however, notice a significant reduction in buzz from flourescent lights and such.
Excellent article describing how to shield your Stratocaster (or copy) from interference hum using a soldering iron, strong adhesives and some heavy-duty aluminum foil. The site has a number of other wiring modifications you can do to your guitar but this one is by far the most useful.  
To shield a Telecaster, use these instructions instead.