How do I stop my bike chain from falling off?

[Myles]

I purchased a 2009 Marin Stinson 17" Bike from REI About two days ago. On my first ride home, I was stopped at a light and attempted to pedal, the bike went no where. I realized the chain had become dislodged, I brought it back to the store immediatley to the bike department. The bike mechanic fixed it and told me that this is typical of brand new bikes within the first 100 miles because the spring is so new it springs the chain off. I took his word for it... fast foward a day later im riding home doing about 15 MPH down a hill and it happens again. I am thrown off my bike and slammed on the sidewalk, right now my wrist is throbbing and the bike sustained a decent amount of damage as well (reflector snapped off, front break broken, scratches, etc.). My question to you bike experts is 1) Why did this happen? 2) Im assuming the technician in the store wasnt honest with me? Please drop some knowlege as I am under the impression i've been sold not only a non functional product but one that puts me in harms way.