I have a mental list of all the trips our family wants to take in the next few years. Every so often, I check airfares to those locations to educate myself so that I know a good fare when I see it. (We went to Paris in March. I had been watching air fares for about 6 months; last July I saw a great fare and jumped on it. As it turned out, in the intervening 9 months until the trip, there was never a lower fare.)
We are a family of 5, so the first thing I do for ANY specific trip is to research accomodations. There are some places where there just aren't good alternatives for a family of 5.
For our late May/early June trip to WDW, I booked a split stay at YC and CR about 3 weeks ago. Even at that, it was too late to get AAA rates at the YC!
I'm reading the Unofficial Guide. From that I made a chart of the days we'll be in the World and have decided, for the most part, what parks we'll visit on what days. After that I started looking at the boards and the UG for restaurant recommendations. I've settled on 4 PS that I want for sure. I've got the dates to call for PS on my calendar but that's not until late Feb.
I'm trying to break in 2 pairs of shoes to wear in the parks. That is something that will take quite a bit of time. (Sorry the kids can't do that, but their feet will grow!) I've also bought my daughters about 8 new camisoles/tank tops each for the trip (to minimize laundry) at end of the summer sales. I got some at Justice for $2.99 each and a lot at Kohl's 80% off sale for $1.80 each!!! I'd love to be able to buy shorts on sale, too, but sizes are more of an issue with shorts.
Last weekend I labeled envelopes for Mousekeeping Tips and each kids' spending money. Next pay day I will have their spending money saved, so I'm going to go ahead and fill their envelopes.
I keep a packing and grocery list on my desk at work, and jot things down as I think about them.
It may seem insane to be doing some of this so far ahead, but we are at a lull as far as kids' activities are concerned. I know that next spring before the trip we'll buried every weekend; the weekend before the trip is Dance Recital Weekend, which is pretty much 2 8 hour days; two of the 3 kids will have finals that week, then school will get out, then we'll hop on a plane. Anything that can be done now is getting done now!