Source: Martha Stewart Weddings
Bridesmaids -- an important part of a wedding as well as of the bride's life -- should be put in touch with each other early and then kept abreast of all the wedding plans as they develop.
Photograph of bridesmaid dress
Swatch of fabric from bridesmaid dress
Photo or sketch of bridesmaid shoes
Send out a letter complete with all the fine points they will need to know: Begin by listing the names, addresses, and phone numbers of all the bridesmaids (they will be thankful when it comes time to plan the shower).
Also include a photograph of the bridesmaid dress, a swatch of the fabric, any information about fittings, and a photo or sketch of the bridesmaid shoes. Finally, a few fond words from the bride will make them feel special -- as all bridesmaids should.