I've seen tons of great ideas for countdowns here.
We just had a calendar that the kids put stickers on, but we had Disney family night once a week for the few months before our trip. We picked movies that had an attraction in the parks. (It was our first trip and my kids had not seen most of the movies.) The kids really enjoyed having dinner in the family room and all watching a movie together. There's even a thread on here that includes themed meals and games to go along with the movies. We've also watched the Disney planning DVD about 100 times.