Maggi has an interesting post about conducting weddings. I remember swapping wedding jokes with others on a clergy conference - they're useful sermon starters (worryingly, no-one got mine). I really believe that the role of the priest at a wedding is to lead the celebrations, and nothing turns me off more than going to a wedding and hearing the priest smugly opine that 'it's nice to be marrying two Christians for a change' (as if Christians were somehow 'more married' than others).
As well as this note of celebration, I try to put three things in my wedding sermons:
1. A reference to the fact that their marriage is for the benefit of others as well as themselves.
2. A reminder to family and friends that they have (at least in the CofE service) promised to support and help the couple.
3. A mention of the way in which marriage reflects the purposes of God in renewing the whole world.
Then it's just down to my bad jokes!