I have seen a number of small group models set up and in operation - helped with several of them.
From the old COGOP ABM "bands" to family groups, to age groups, to day of the week groups (Sunday night group, Saturday night group, Wednesday night group), etc.
In fact, here at our church, our pastoral staff have attempted several of these various models over the years.
What works for us, may not work in other places or churches. But, here is what we have found works best for us. We do still have some old time Nazarene folks who like the ABM style of group (I thought it was a "unique" model that COGOP invented

), but they only attract folks over 60 who grew up with that model.
Our small groups are set up according to interests. If there are 2 people with the same interest, they are encouraged to start a small group and invite neighbors or others who are interested in the same thing.
Our small groups are encouraged to stay on the small size of around 12-15 people. We feel this encourages:
1. More closeness and intimacy for connection purposes.
2. More confidentiality for sharing our failures and shortcomings.
3. More accountability - definite necessity for our size congregation.
4. Easier for us Lay Pastors to stay in touch with so we know the needs and can communicate them to the paid pastoral staff, as needed.
When a group gets over 15, we ask that one of the group members branches out with another group - like cells grow and divide - so the body can continue to grow and multiply.
Our small groups operate on a 13 week semester basis. We attempt to start a new study, book, program, etc. in each group every 13 weeks. The model we use is to study together for 12 weeks and then do a "social" event on week 13 - pot luck; attend a concert, play, etc.; go on a sightseeing trip, etc.
Our small groups include (but are DEFINITELY NOT limited to): choir, audio/visual ministry, stamping (they make cards for the pastors to use for visitors and for visitation (hospitals, etc.), cooking, motorcycles (our largest group meeting away from the campus - about 150 folks), various books - we usually have anywhere from 25-50 different books being studied on any given week (except right now because most of our groups are studying The God Questions to coincide with the sermons), learning guitar, computer training, grounds maintenance, facility maintenance (yeah, there are 2 separate groups), Coffee Shop ministry, DVD/CD ministry, nursery care for the local AA and NA groups, hunting, fishing, 4x4, paint ball (hey Rambo), Mexico Missions, Africa Missions, Portugal Missions, High School, Junior High, Prime Timers (over age 50), Rescue Mission, praise band, etc. and etc.
Hope this is helpful even though I'm only a "Lay Pastor".
