How do I stop my dog from eating things off the street when we are out walking?
http://uk.answers.yahoo.com/question/index?qid=20110130131240AAgRJqn
I saw on a show that there is one way to correct this behaviour.
Take treats with you on your walk, or you can even practice this in your home.
Drop something that is very tempting to her on the floor, then when she goes to acknowledge it, redirect her attention to the treat in your hand. Keep doing this until she basically knows that she's going to get the treat for leaving the thing on the ground alone.
It might be helpful to do this outside as well since that's where you are having problems. Just be consistent and don't give up. You really need to teach her that it's not okay to eat things that she finds.