It's hard to believe, but there are several times of year when the crowds at Disney World are very manageable. The month of September happens to be one of them. The Princess and her Prince Charming are there now and she keeps texting me about how short the lines are (they rode Expedition Everest yesterday because the line was only 5 minutes long and they had some time to burn before Flights of Wonder.
From around September 1st to Thanksgiving week, for the most part are slow crowd wise. Of those, if you go the first week of November, you get to see Halloween come down and Christmas go up. It's the best time of the year in Dad's opinion to see the most at Disney World.
The best place to get crowd predictions is at Touringplans.com. They look at each park day-to-day for crowd levels. They have years of history to back up the predictions. I'd start by looking at their projections to pick the best time to visit.
Cary, if it were me (and it is) who was a Senior Citizen who wanted the least amount of crowds at Walt Disney World but still wanted the best experience available, I'd go in early November. But I really love Christmas at Disney World. My second choice would be October then January.