Emotional reinforcement is just as important as on-the-ground, tactile support like setting up chairs or mailing out invitations. Think of yourself as a psychological security blanket for your friend. For example, reflect on what she says (and doesn't say) before firing off your opinion. Empathize with her point of view and give thoughtful feedback when asked for input. Focus on what she feels and wants, as opposed to what you think she should feel or want. Approach delicate topics with sensitivity, and above all else, keep a positive attitude.
Remember, there are probably numerous points of stress for the bride, from travel logistics for guests to appeasing a volatile mother-in-law. She's got a lot on her mind: getting gown alterations done on time, tracking down the videographer who's late or worrying about the weather report. Be her rock by staying calm, patient and upbeat.