Generally, the processional should include songs that are performed at a tempo that is easy to walk to. The procession is very much like a march, and you have to make sure you and your bridal party can keep up.
If there is an elderly person somewhere in the bridal party. (e.g. your Grandfather is giving you away) is the music you choose something that person can keep up with? Will the Flower Girl and Ring Bearer have to run to keep up with the pace the others in the bridal party are keeping? How quickly can the bride really move in her heels? If you are going to err, err on the side of slow. Make the procession more of a pageant than a parade.
2) Length and Flexibility of the Songs
Your DJ/Musician(s) can usually help with this but, in general, you want to pick music that can be readily, easily, and smoothly ended at various points in the song. This is to accommodate the length of your aisle, the number in your wedding party, and the speed with which they will walk.
It's a good idea to practice using the music you want during your rehearsal. This will give your DJ/Musician(s) a chance to practice different places to end the music. Even with practice, it is possible that the folks in your processional will move at a different pace on your big day than they did in the rehearsal. This makes it even more important to choose the right music and a DJ/Musician(s) with a good sense of timing - you don't want to have to wait too long for the music to end after the bride reaches the front of the assembly.
3) Number of Songs
Most processionals include either one or two songs. Some couples decide to have just one song for both the bridal party and the bride. (this is especially appropriate for small wedding parties and situations where there is a small or no aisle). Others choose to have two songs - one for the bridal party and one for the bride (this is especially appropriate if the bridal party is large).
As mentioned elsewhere on this site, the bride's processional itself could also be two separate pieces of music. Sometimes, there is a piece called a "fanfare", that heralds the arrival of the Bride.
In addition to these two or three pieces, some couples choose yet another song for the Ring Bearer and/or Flower Girl. This is a matter of taste but, in general, you probably wouldn't want to have any more than four separate pieces of music for the processional - and this is especially true if the aisle is shorter, and not conducive to a long processional.
4) Contrasting Song Styles
In the case where two different songs are used for the bridal party and the bride, a contrast in style or genre between the two songs is recommended. The change of styles helps to draw a psychological/musical contrast for the guests, and highlights the appearance of the bride.
If a fanfare is used it is, by its very nature, a different piece of music. A fanfare is a short (10 to 20 second) but lively/rousing air played on brass (usually trumpets). It immediately precedes the bride's processional song, and helps to create a dramatic effect just before the entrance of the bride.
Finally, if you choose a different piece for the Ring Bearer and/or Flower Girl, this too would be of a different nature than the any of the other pieces - perhaps something a bit lighter, or a song that mentions "angels", etc.
5) Religious Processional Music
If your marriage is taking place in a church or other religious venue, you'll need to discuss your music choices with the person performing your ceremony. In some cases, you'll need to use religious music for the processional songs, as opposed to secular music.
If you are planning on recorded music, you'll need to check to make sure that is acceptable. Sometimes, you can have your DJ/Musician(s) play up until the bride's processional, which is then played by the church organist.
Different religions and denominations have different rules, and different sites may have different rules even within the same religion so, again, make sure to check the rules with your Celebrant before you make solid plans with your DJ/Musician(s).
As mentioned a bit earlier, it is appropriate to choose two different processional pieces for the bridal party and the bride. So next, let's check out some great songs for the bridal party's processional!