How do you weatherproof a light switch?
If National Electrical Code applies, here's what it has to say...

National Electrical Code 2011
Article 404 Switches
404.4 Damp or Wet Locations. A surface-mounted switch or circuit breaker in a damp or wet location shall be enclosed in a weatherproof enclosure or cabinet that shall comply with 312.2. A flush-mounted switch or circuit breaker in a damp or wet location shall be equipped with a weatherproof cover. Switches shall not be installed within wet locations in tub or shower spaces unless installed as part of a listed tub or shower assembly.

Which means that if the box containing the switch is not mounted inside the wall, you'll need a weatherproof box.

Once the switch is mounted in the box, you'll need a weatherproof cover.  You'll have a couple options here. You can get one with a flip up lid, where the cover has to be flipped up before the switch can be toggled.

There are also covers available that completely encase the switch, and have an external lever that is used to toggle the switch.


Bonus
Yes, you can supply a switch from the Load side of a GFCI receptacle to provide GFCI protection to the switch.