We "scheduled" our next trip (for youngest DD's 10th birthday) on the way home from our LAST trip (which was her first, and for her 5th birthday.) We're going next June after school lets out, and I've just started my "serious" planning ie- chose dates and started the strawman itinerary.
Last trip I spent about 18 months on it, this trip, it's more like 13 months, and only because that's how far out the rentals open up on the condos, and we want a good selection. It's also how far out you can book the auto-train, though we're not doing that, because the cost is simply not worth it this time, so we'll be doing the long-haul drive. I feel a little behind the curve with all the changes, but luckily I've got the good folks of the DIS to clue me in...
My parents are taking their RV to Fort Wilderness and are waiting for the dates to unlock, which will be very soon, and also about a year out.
I can't tell you yet which park we're going to on which dates, but I can tell you what/where we'll be eating the day we go to certain parks...

Once the reservations are locked in and we have an idea of park hours, and EMHs (we don't use them being offsite, but my parents might, meanwhile I want to dodge the crowds, if possible) then we'll try to firm up which days we are going to which parks, so that we are ready to try for our ADRs at 180 days out.
Meanwhile I have a mental list of things to acquire in the next year. I had great success last time with finding useful stuff (a lot of it NEW) at yard and rummage sales... that helped me add a lot of magic and still be in our budget.
I just bought one of those "photo boxes" from the craft store (Frozen themed) and have started the "gathering" process. I'm also not above putting stuff for the trip on my Christmas wish list or giving trip-things as Christmas gifts. ;D I've started actually clicking on the MyPoints emails and doing swagbucks half-heartedly. I'm not going to "work" at it, but I can probably get $25 out of each of them by next year... that's free snacks...
I agree with what so many others have said. The planning is a big part of the fun!