We did a battlefield tour a few years ago, with Julian Humphrys, then working for the National Army Museum, the tour was excellent, the museum was ok, but you'll be hard pushed to find out why Boney lost, there were some Brits and Prussians involved, but basically the frog rolled up to fight, then changed his mind. Lots of Napoleonic busts and such like, but little mention of the fact, HE LOST! AGAIN!
If you can manage it visit the Sibourne Model at the N.A.M in Chelsea, it gives a bird's eye view of the battle and helps relate the battle to the ground when you get there. The museum as a whole is worth the time, and it's free.
Trivia, the Lion's Mount was built by taking much of the earth from the battle field to build it; Wellington said something along the lines of, 'They have destroyed my battle field!'
Trivia, the teeth of the dead were harvested to make false teeth, and for years afterwards false teeth were known as 'Waterloo teeth'