We generally go to Disney once per year at about the same time, but still take advantage of the savings on annual passes. We schedule our trips 50-51 weeks apart. For example, in 2009, we might go 12/12/09 - 12/19/09. Then in 2010, we'd go 12/4/10 - 12/11/10. We'd get the AP (or excahnge the voucher for the actual pass) on 12/12/2009. That way the AP would be good for both trips. Having an AP also allows us to get the Tables in Wonderland dining discount card. That saves us 20% off food and beverage in most Disney TS restaurants and costs annual passholders $60.
For your family, you'd have to do the math depending on your own itinerary. Will you want to park hop? Will a 6, 7 or 8 day pass be less expensive (you'd need the no expiration feature) than two 3 or 4 day pases? Will you want to visit any of the waterparks? The Magic Your Way tickets are very flexible, but that flexibility means you really have to plan and do the math to find the best option for your family.