Book your hotel?
Book your flights?
Make dining reservations?
Start packing?
As soon as I can get an unbelievable deal. The longer out you book, the more chances of making changes if a better deal comes along.
I use travelzoo.com to keep track of airfare - they send me a weekly email with all the latest deals. They don't last long, usually the best are sold out within 24 hours. I started getting better deals when I signed up for the VIP newsletter, it also comes a day earlier (usually Tues. evening). Spending the xtra $15 for insurance allows you greater flexibility if you need to cancel your flight.
Hotel: a year in advance if possible - we booked with the homeschool conference back in November for a September trip. The group has rates of 195 per night for the GCH, and 169 per night for DL. Those dates are only for September, so check on the site to see if they work for you:
https://resweb.passkey.com/go/swhe09
Dining: About 2 weeks before if needed, but we don't do anything where priority seating really matters. Key to dining more comfortably: eat earlier. We eat around 4pm. While everyone else is standing around waiting for a table at "dinner time", you can ride the rides! Usually, we only eat breakfast and dinner, and buy a piece of fruit or a bengal BBQ stick during the day. This leaves more money and time for a nice dinner.
Packing? lol. usually the day we leave, we're not that well organized. It's more important to make a list than to actually pack. We will start lists weeks before the trip, and adjust them as we get closer. Then, the day of, just make sure all the laundry is done and it takes us about 15-20 minutes each to pack by running around with the list.