This is a very timely question a) because the holiday season is approaching (and group shots are very popular); and b) I'm taking a portrait photography course from an amazing teacher who specializes in group photography. You can check out his work (lots of great ideas) here:
www.nealslavin.com and an interesting, recent interview with him here:
Neal Slavin: Why I Photograph Groups - L'Oeil de la Photographie
Others have mentioned this, but the main thing I'm learning about photographing groups is how essential it is to prepare (as much as possible) in advance. When we do our group shoots, we set up lighting and test it with stand-ins first. We try to find out how people usually dress, so that we can anticipate lighting challenges. (For example, the other night we photographed in a studio setting a small group of rabbinical students, who were going to be wearing very dark suits, white shirt, dark hats and have facial hair. Obviously, we needed a fairly wide swath of light with some fill on the other side in order to ensure that all faces were lit.) We use stand-ins to test how close we need to bring people together. For a very large group, we don't use 1-for-1 stand-ins, but, for example, for a group of 20 or so we might try to practise with at least 6-8 people if we can. In your case, you could ask for a few volunteers to serve as stand-ins as you practice - and you might even want to practice the day before (at the same time of day), if possible. Otherwise, plan to work on set up for at least an hour.
I can't add the link from the computer I'm typing on now, but if you go to the Facebook page Neal Slavin Photographer and scroll back to last week, you'll see the end result of our rabbinical students photo shoot. I think the image is worth the PITA factor of digging around on FB for it.
When you're composing a group photo, bring people onto the set in small groups. For example, in a group of 3-5, ask one person to step in and then invite others one or two at a time. For your large group of 30 or so, have people come onto the set in groups of 5-6. There are at least two reasons for this. One, you want to watch carefully the dynamics among the people - which will be more obvious if they organize themselves a few at a time. You'll see who the leader is (or leaders are), how people want to relate to each other, who is the joker, who is shy, etc. And if you're outside in less than ideal conditions, bringing them out in small groups means that you're exposing them to conditions for a shorter period of time.
I noticed that it was helpful when we were shooting groups to have two people (photographer and assistant) involved in the shoot. This is because one or the other of you is usually going to be concentrating on one thing (e.g., getting the lighting right, moving people around) and because of that might be less observant than usual. The other person can observe the group and make suggestions about approaches to the shot. It's definitely one of those situations where two heads are better than one because there's so much to pay attention to.