Traditional responsibilities of the maid of honor include:

Helps the bride select bridesmaids' attire and wedding dress
Helps address invitations and place cards
Attends as many prenuptial events as possible
May host a shower, but not mandatory
Organizes bridesmaids' gift to the bride and usually gives an individual gift to the couple
Pays for own wedding attire and transportation to wedding
Attends the rehearsal and rehearsal dinner
Makes sure that all bridesmaids, flower girl, and ring bearer are at the rehearsal and the ceremony on time
Walks in processional and recessional
Holds the groom's wedding ring
Helps with the bride's gown
Arranges the bride's veil and train before the processional and recessional
Makes sure the bride's gown is picture perfect throughout the day
Holds the bride's bouquet during the ceremony
Witnesses the signing of the marriage certificate
Stands in the receiving line
Keeps the bride on schedule
Helps the bride change into her going-away clothes
Takes care of the bride's gown and accessories after the reception