Below is a list of the most common duties performed by the bridesmaids.  

Print it out and give a copy of it to your bridesmaids.

Duties before the wedding:

Encourage and help the bride plan her wedding.
Help decorate ceremony and reception locations where necessary.
Help the bride select the bridesmaids' dresses.
Help organise the hen's party.
Help prepare and send the wedding invitations.
Help assemble the souvenirs.

Ceremony and reception duties:

Offer to help the Maid of Honour when needed.
Always be prepared for the unexpected so that no upset is caused by things that happen through chance.
Stay close to the bride throughout.  Never allow the bride to waste valuable time looking for you when she needs you.
Help the bride and younger bridesmaids get dressed on the wedding day.
Assist the bride in and out of the wedding car, taking care of the dress and veil.
Ensure the bride looks her best at all times and adjust her dress and veil when needed.
Stand in the receiving line (if there is one.)
Sit at the head table in the case of a 'sit-down dinner' reception.
Collect the gifts and give souvenirs to guests who did not collect theirs earlier.
Help the bride into the reception and attend to any other task she may require.
Help load the gifts into car.
Help with any clean-up and collect remaining gifts at the end of the reception.