Doing good animation isn't just about planning ahead. It's about doing your research. Often I'm surprised by how something moves in real life. One of the best examples about video reference is with the bird in Pixar's: A Bug's Life. Apparently a bird was flying around outside the studio in Redmond. One resourceful animator grabbed his video camera, ran out and video taped the bird. That one videotape bounced around all the animators who animated the bird in the film. The results were fantastic! The bird could have been animated in a hundred different ways, but I think the refrece helped the animtors pick a way that had perfect "True To Life" timing. When you watch it it just feels right.
If your looking for video reference on the web, here's a couple that I find useful:
The BBC has released their stock video collection on line. You have to sign up for a free membership, but it's worth it. They have tons of reference video. You can even buy the photage if you'd like:
Also Yahoo has a just started a video search. I find it can be useful:
If you know of more please let me know!