We wanted to start planning as soon as we got back (2008), but we planned another adults only trip for last month. Now, we're planning to go back again. The biggest hurdle for us is airfare, which is half the total cost for family of 4.
You can loosly plan, but until you experience it, there's no way to gauge. Even after 2 times, there still stuff I have missed, and stuff I could care less if I see again. Different trips can have different focuses, like character hunting, extreme rides, or some combination. The kid's ages will help steer you to or away from certian rides/attractions as well.