A few postcards or letters from the Trail mentioning how their advice helped, and a photo when you finish, would also be a great way of showing your appreciation. Read books about thru-hiking. Many good books are available to help you prepare, from memoirs of past hikes, to planning guides in workbook format. Contact a hiking club in your area. If you live on the east coast, chances are there is a local A.T. maintaining and hiking club in your area.
Many clubs have members who have thru-hiked.
Also, going out on some work trips before your thru-hike will give you a tremendous sense of satisfaction and an appreciation of the work that goes into building and maintaining the Trail.