Budget is my main planning thing right now.
We have been saving for our summer 2007 trip since the beginning of this year. A little goes away with every paycheck. We don't even see the money, it automatically gets transferred over to a dedicated savings account on payday. And as long as we don't have a very serious emergency, we will have more than enough for the 5 of us to go (me, DH, DS-who will be 4 at time of travel-and 2 sisters in law-who will be 11 and 13).
As for other planning, we know when we want to go, where we want to stay, and how we plan to travel. While 2007 prices aren't really out yet, I check every few months just to see if we are still on budget (i check with equivalent 2006 dates and just figure it will cost a few hundred more than that-taking into consideration that there will probably be increases for 2006 and again in 2007).
Beyond that, I don't plan on doing much heavy duty planning until we tell the 3 kids we are going. They will find out Christmas of next year (2006). I plan to give them each a packet of "stuff" they can look at and help to decide where we eat, which parks, which days and all of that. I want them to have some say in what we do while there (especially since the girls will be babysitting the 4 year old throughout...that's why DH and I are paying for their trip...).
I will probably book in January of 2007 for the July trip (I know, I'm nuts, but that is really the only time available to us to travel). I'd love to be able to get a great deal, but I am more concerned with locking in a room before they are all booked up. And as long as it doesn't go too far over our budget, I'll be happy!