The first time we went, DS was 3 months shy of turning 3. We prepared him the entire year before the trip by renting lots of Disney videos especially the ones that have attractions modeled after them like Lion King, Toy Story, Bug's Life, etc. We also watched the planning videos and the Disney Sing Along videos.
Disneyland Fun is the best because it shows many MK attractions. We talked about the rides, etc. and made a list of what he wanted to ride and see.
Also warn your child that the characters are HUGE and they don't talk. Our DS is very shy, but he wasn't scared because we had already warned him. It's fun to start off the trip with a character meal like Chef Mickey's to see the characters before the overstimulation of the parks. He still treasures his autograph books.
We always did MK early in the morning when he was fresh and the lines were short. The other parks didn't seem as crowded later in the day. Epcot is nice in the evening because your child may fall asleep in the stroller and you can enjoy the ambience of WS.
Don't forget the camcorder. It may be a pain to carry around, but the video is so fun watching when you're missing WDW. DS prefers to watch our Disney trips than his other videos.
Earplugs are a must for the loud shows and a penlight doesn't hurt for the indoor attractions.
We have a 3 month old now and can't wait until he's old enough to go! Enjoy!