In your throttle body there is a butterfly valve that is opened and closed by you pushing on your gas pedal. Over the years this valve gets gummed up and sticks. The solution is to simply clean inside the throttle body.
You can have this service done at a shop or if you are handy, you can actually do it yourself.
I found a pretty good description of how to do this at this site, if you'd like to try it yourself.
〉 Answered on Dec 21st, 2008 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines