My second job in full-time ministry was a busy north-eastern church where there were four or five of us on the staff team who could take weddings and we had many weddings - as many as five on a Saturday between April and September. We did wedding duty. You took one weekend a month and did all the weddings on that day. You could be very lucky and have none or very unlucky and have five (I did once), or have a bank holiday weekend with a Friday wedding, a few Saturday ones and a Monday one too.
The choir got to know my standard wedding sermons for non-church-going families pretty well. There was:
John 2: 1-11 - the wedding at Cana. Basic mesage - Jesus makes parties go better.
1 Corinthians 13 - the hymn to love. Basic message - imagine a church where things had gone so wrong they had to be told off for not loving each other.
Psalm 145 - good to acknowledge God in the middle of special occasions.
Colossians 3:12-17 - love binds all things together (similar sermon to 1 Corinthians 13 but more able to do justice to the passage).
I tried to do a different one for each wedding on the same day. Sometimes I'd let the choir choose.
If I knew a couple well I would encourage them to pick a different passage and then use a one-off sermon just for them.
My attitude in preaching was, and is always, that I was speaking to the couple and inviting the witnesses to overhear.
There are some good wedding illustrations, quotes and jokes. Many get over-used. If I hear a new one I tend to us it for a bit then discard it. I often tell a story about my own grandmother because I know nobody else will use it, or tell it quite as well.
There is a good Grove booklet on preaching at weddings (Grove Worship 74) by Ian Bunting who was the rector of that same north-eastern church before I got there.
These days I know my four wedding sermons off by heart so don't use notes at all. I try and judge the atmosphere. Most guests will expect you to be dull and long-winded. Being interesting, spiritual and punchy will surprise them.
There is no point in having more than one point.
I sometimes print off the sermon I used and send it to the couple with a card after the event with a note that I am there if they need me. Then I leave them alone.