Make your friend feel special leading up to her big day. Especially around her engagement and during the planning. It's an exciting time. Show the Bride how excited you are for her.
Make yourself useful. Help her get items checked off her list. Don't just tell the Bride to let you know if she needs anything, offer to do a specific task to take it off her hands entirely.
Try getting along with all the Bridesmaids. You may not know each other very well, but working together is invaluable leading up to the big day.