Gee...I was in the middle of typing and trafficgoo posted. Mine is the longer instructional version.

For what it is worth, here is my reply:
BizzyB53...
The way I do it may be different than others, but I will attempt to explain below. The key is to find out who the longest married couple will be attending and whether they had a "special" song at their wedding. If no wedding, a very special meaningful song. Try to obtain it and make it the last song in the series of a 3-song set.
During the evening prior to the Anniversary Dance, be sure to make an off the cuff announcement, well in advance. Something like, "Gentlemen, later in the evening you are going to need to know the NUMBER of YEARS tha you have been married, so if you aren't sure, then you had better check with your spouse."
The Anniversary dance, as I know it.
Gather all married couples on to the dance floor. The newest married couple (Bride & Groom) included. Have three songs ready to play. Give them a brief explanation of how this dance will progress.
This is the Anniversary dance. Remember when I suggested that you men should confirm the number of years you've been married? Well, now is the time to know.
When the music starts I will be calling out numbers that correspond to the number of years of marriage. When your "number" is called, please move to the edge of the dance floor. At the end of this dance there will be two couples left in the middle of the dance floor; the longest married couple and the shortest married couple (Bride & Groom). So let's get started.
The three songs should be representative of those who have recently been married (6 mo - 5 yr), then middle age (6 yr - 20 yr) and then long-term (21 yr - ?).
You will need to switch songs as the dance floor becomes less crowded and end up with the song that may be from the longest married couple's era...or perhaps "their" song.
When the last two couples are left on the floor. Stop the music and ask everyone to congratulate the longest married Bride. Then the new Bride will present a small "bouquet" of flowers (usually carnations) to the longest married bride. You can then ask the LMC if they have an words of wisdom for the young married couple.
There may be other ways of doing this dance. This is the way I learned of it. Of course, you need to discuss this dance with the B&G in advance to determine if it would be worthwhile. It works best if there are going to be plenty of married couples at the wedding. The number of years of the longest married couple is really not significant whether it be 40 or 50 years. I would offer a suggestion about couples who have been married for a very long time and don't have the stamina for a 3-song set. You may want the Bride to ask them to come to the dance floor when the third song begins so that you don't wear them out before the end of the last song.
Hope this helps.