Host or Co-host a bridal shower with the help of the other bridesmaids and/or the bride's mother, sisters, etc.
Help the bride choose her dress and shop for bridesmaid dresses. (The Wedding Expos is the perfect place for this one!)
Make sure the bridesmaids all go for their dress fittings and get everything they need for the big day. ie jewelry, flowers, shoes, etc
Lend an ear when the bride wants to/needs to vent, no matter what the topic!
Generally keep the bride sane during wedding planning. Can help with addressing Invitations, choosing colors, cake, decor, attend the Expo...
Make sure the bridesmaids know where they need to be and at what time on the big day. Also, inform the bridesmaids about all prewedding events and parties.
Plan the bachelorette party with the other bridesmaids.
Keep a record of all gifts and give the list to the bride for Thank You cards.
At the ceremony, stand next to the bride while she exchanges vows and hold her bouquet during the ring part of things.
Stand in the receiving line. May be announced with the Best Man.
Bustle the bride's train for the party. Also, help the bride change clothes for the honeymoon and take charge of the dress until the bride returns.
She may dance with the best man during or after the couple's first dance. Also, can give a toast after the best mans'.
Generally keep the bride sane during the wedding itself and help her truly ENJOY the most wonderful and anticipated day of her life...so far.