I've gone in 1976 (class trip, don't count that one) and 2002. For the 2002 trip, I started saving and planning about 18 months out, maybe a little more. Wanted to take DD for her birthday. I have relatives and friends who can go wherever they want, whenever they want. I am broke

usually, so I had to plan ahead and save. That said, as it got closer the planning was more fun.
I started by reading up on Disney and lurking on the boards since this was basically our first trip, you can't compare 1976 with 2002, different world. We started thinking about what attractions we wanted to see.
I added up everything we needed (room, air, spending money, towncar, even new shoes) and then figured how long it would take me to save for the trip.
The first thing I did was reserve the hotel. I tried to reserve Pop century but since the opening was delayed (I think that was it) they offered me geat deals on moderates or AKL. So we went with AKL.
Then I made a list of everything we needed to do and checked off each thing as it was done. (Buy plane tickets, reserve towncar, buy park tickets, schedule dog boarding, first day we could make PS arrangements, etc.)
I don't know if the
disney store still has disney dollars, but I'd buy a few of those when ever I got the chance.
About 4 months before we came up with a list of what we really wanted to see and how we would approach touring. We were thinking about this probably the entire time anyway. But I wrote down what day we'd go to certain parks.
As the date approached (2-3 months ahead) I started renting all the old disney movies to get back in the mood after such a long planning and saving timeframe.
1 Month before - I made a packing list so I had time to get anything we still needed.
With each milestone we reached (room, tickets, plane tickets) we'd get more excited about the trip.