"Trouble" doesn't want to stay in the yard!

We have an 18 month old female lab/border collie mix. Her name is "Trouble" and she has so many endearing qualities and we love her dearly. She is learning to sit, stay, and heel and doing a pretty good job of it. One of the few problems we are having is keeping her from bolting out into the front yard (and beyond) when we open the carport door. We have a second dog, Nikki, who is almost 16 years old and we have never had to worry about her trying to bolt out the door or leave the front yard when allowed out. Don't know why, but she has just never been thus minded. Any suggestions on how to best train Trouble to not be inclined to bolt and/or leave the front yard?