Processionals

You'll most likely want one processional for the bridesmaids, and a different one for the bride.  You'll want the louder piece to be for the bride.  A classic combination is "Canon in D" for the bridesmaids, followed by "Trumpet Voluntary" for the bride.  If your wedding party is very small, it is also possible to have just one processional for both the bridesmaids and the bride.  A Medieval sounding pairing is the "Glorificamus" followed by the "Agincourt Hymn" found below.

Bridal Song from Lohengrin (Wagner)
Canon in D (Pachelbel)
Jesu, Joy of Man's Desiring (Bach)
St. Anthony Chorale (Brahms)
Trumpet Voluntary (Purcell)
Canon on a Theme from "Glorificamus" by John Redford

The Agincourt Hymn by John Dunstable

A more recent trend in weddings is to use a song instead of an instrumental piece.  This piece was written to be used in that way, with the bride entering on the second verse where the veil is mentioned.