Our family (DH, DS11, DS9mo) heads to Disney at least once a year. We get very excited about the trips and I think that has rubbed off on our families. They generally wish they were going with us on each trip and have in the past almost joinned us at the last minute. Excitement is very contagious in our case.
We have planned a 1st birthday trip to WDW in October for our youngest and have invited our parents and my sister along. They are all getting excited already with
79 Days 6 Hrs 38 Mins 58 Secs left before we leave. I have been sending out occasional e-mails with the countdown to our trip and include trivia, pictures from our past trip of us having fun, places we will be visiting, etc. The e-mails are usually fun and entertaining and they have really enjoyed them. Whenever we get together we usually bring up the trip and usually always ask if there is anything they would like to be included in the plans. Letting them be a part of the planning tends to get them excited and keep them interested. Be careful with this though, it may open another whole bag of worms...(for planning natzies). Since I am the planning fanatic and have planned something for everyone, they have only come up with Whispering Canyon (which I actually already had planned

). They sugessted it because of all the stories we have told them from our past visits.
So I suggest getting

about the trip yourself and hopfully they will follow!
Disclaimer... Do try to keep it subtle so you don't turn them off to the idea. Our parents have been through years of conditioning to our Disney obsession and really aren't phased by our banana dance any more...
Edit: DH has informed me that it is a "can of worm", but I believe I prefer the bag...