While it occurs to me, I thought I'd share the "what I learned" part of researching and booking the safari. That will, after all, give me another reason to talk about it.
First, set a budget, both financial and schedule. How many days do you have, realistically? How much can you afford to spend and do you really want to spend that much? Look at airfares beforehand, and build in the cost. Know what airfares leave when and build your dates around what appears to be the cheapest fares, if you can. Realize that it takes a whole 24 hours to get there and another 24 to get back. Those days eat into your budget.
Next, identify what you want to accomplish. Are you going all for the animals? Do you want time on the beach in Zanzibar? Do you want to spend time in a bigger city there? We wanted all animals all the time, with the exception of a visit to a Masai tribe and a cultural visit like a school or orphanage. We'd want to also do something charitable.
Now that you have all that in mind, start looking for providers. If you're fortunate enough to be part of a small group, price out a private tour. Look on the internet forums (Safari Talk, Fodors, Trip Advisor) for reviews and postings on particular tour operators. Based on discussions I had with many travelers who'd gone before me, I knew that the experience would be better and cheaper if we went with a tour operator who is based in Tanzania and employed their own guides rather than contract for the guides. I limited our research to about five of the most frequently recommended tour operators. There are many US-based tour providers that include Tanzania among their many offerings, but I felt like that was more a "jack of all trades, master of none" scenario. I wanted someone who only worked in Tanzania and was a subject matter expert on Tanzania.
Talk to each operator. If they'll only do business by email, forget it. Only through discussion will the tour operator be able to get a sense for you and your group. The operator we ultimately booked with even offered to do a conference call with all of us travelers. Once they have a feeling for your goals and aspirations, let them put an itinerary or two together for you. Vet that itinerary among your group, including proposed accommodations. Find reviews on the accommodations and make sure they match your travel style. We knew we weren't camping in tents and sleeping on the ground types and we require real beds and real facilities in the bathroom. That is what we requested and what we got.
Agree as a group to extras, like sunrise hot air balloon ride, cultural events. Some of these add (substantially in the case of the balloon ride) to the overall cost.
Talk to as many references as you can find. Our operator provided us with a reference list and I contacted them and travelers on the forums who mentioned they'd traveled with this operator. Across the board, I heard the same thing, same satisfied customers. It became almost boring to hear the heaps of praise, but it was after 7 or 8 such discussions that I realized we had found our operator.
Check your airfare again before you put a deposit down. I'll admit, this is where I/we went wrong. By the time we booked, I realized the cheapest airfare didn't mesh with our schedule, and we ended up paying more for it than we hoped because we had to take a later flight on a Friday, which is the busiest day flying out of Kilimanjaro.
Put the deposit down and book the airfare asap. Now sit back and breathe...
For me, the planning was the fun part but for this trip it was immensely time consuming. I would estimate probably 25-30 hours of research, 10 hours of reference calls and then time spent relaying information to the group. Don't get me wrong, I loved doing this. It just required a lot more brain power and energy than most other trips.