We also put money in a savings account every month that is strictly for vacations. Any money DH and I get for our birthdays or anniversary and any kind of rebate we get for anything we buy goes in the vacation fund, like the $100 rebate we got last year from buying our computer. We also pay for our actual trip in stages.
For our trip next Oct, this is what I've done so far.
1) I bought one adult park-hopper ticket this summer when I was at the
Disney store, then I'll buy another one every three or four months (we need 4 total).
2) I made our hotel reservations a couple weeks ago (on-site), so I had to put down a one-night deposit (we're staying five nights). I'm planning on sending in more money after Christmas, as my in-laws always give us some money. Then, I'll try and send in a little money every month so we don't have much of a balance left right before we get there.
I try to make our plane reservations at least 3 months in advance, but I start looking 6 months in advance. Then the tickets are paid for before we actually go (we charge them, but pay for the bill with $ from the vacation fund).
The only thing we end up having to pay for once we're there is food and souveniers. Our kids are 7, 4 and 1 and we buy them each one souvenier. After that, they have to use their own money to buy what they want. They save up money from birthdays and Christmas. On our last trip this worked really well, they each had $50 of their own to spend. My oldest was 5 and she thought she really wanted a Tinkerbelle costume. Once I told her it would take all her money (this was on the first day we were there) she changed her mind. She bought a few other smaller things instead, most of which she still plays with.