Just some ideas from the couple of times I do it:
- Have lots of conversations with the photographer about how he works and what would help him - look at past wedding pics he has done so you know what he is likely to be trying to do
- If he is attending the rehersal then go with him
- Make sure he gets the help he needs in the first instance
- Second to that, get the shots he doesn't get - no point shooting the group shots from another angle - you should either help with organisation or be shooting other guests.
- With the groups shots it MAY help to either be organising the next group (make sure you have a shot list), or getting people to look at the right camera (if there are guests with cameras), or making a fool of yourself so the kids look in the right direction.
- Make sure you know where he is standing in the church/ceremony and go somewhere different
- Learn everything you can
- Have fun!
Hope that helps