We went on our first trip to WDW in December with 4 kids (18,15,8,6) and am so glad we prepared ahead of time!
I "overdid" it on buying the guide books-Birnbaum's, Unofficial, Passporter, Frommer's and others

but they did help out greatly! Used the Unofficial the most as it had the best unbiased advice to give IMHO
I made up a sheet for each of the kids as to what they wanted to do the most in each park-we watched the WDW planning video and read Frommer's "Walt Disney World with Kids" to them so they had an idea of what to expect on rides.
We made sure that these were the rides/attractions we did first, anything after that was a bonus! I used a couple of the daily plans from the Unofficial guide and they worked out perfect as they are not minute by minute! (MGM and AK) We weren't stuck to any strict planning guide and went about at our own pace!
Before we went, I wanted us to "stick together" and do it all as a family but realized it isn't possible with the varying ages. Thankfully I came to my senses before we got there! We enjoyed it even if we weren't altogether, all of the time.
Occasionally, DH and I would split up, one would take younger kids to a ride while the other took older kids to rides they wanted to go on---the older kids went on all the rides once as they had never been but they needed to go off and enjoy things they wanted to see and do!
Sure we missed doing some things and didn't see all of the top rated shows but the kids got to do what they wanted to, they loved it and it gives us a reason to go back!
When we got home we were overwhelmed

as there is no possible way you can do it all in one visit--the one thing guaranteed is we were glad to have prepared before going---I can't even imagine just going and "winging it"!
So top things I say are "must dos"----
1. Make a list of each family members favorites
2. Ask plenty of questions at DIS because you get the most amazing tips
3. ENJOY!! Don't rush, take your time and have fun!