You really have to neglect a bike for it to rust. And even then its typically the stuff i.e. pedals, wheels, seat post, stem, etc., that rusts and not the frame. The only exception is a Walmart bike. It may look identical to the bike you bought at your LBS when its shiny and new, but a year from purchase is when you'll really see what you get for 100 bucks.

The simplest way to keep in from rusting is to stay out of the rain. If it does get wet, rinse it with tap water as soon as you get home. Even better, wash and dry it.