Wikipedia has a decent article on the 12 basic principles of animation, "a set of principles of animation introduced by the Disney animators Ollie Johnston and Frank Thomas in their 1981 book The Illusion of Life: Disney Animation." I'm just excited because I found a paperback version in a used bookstore for $4 this last weekend.
If you don't have John Lasseter's seminal 1987 SIGGRAPH paper based on The Illusion of Life, there's a decent scanned copy available: Principles of Traditional Animation Applied to 3D Computer Animation.
Fuel Your Motionography finished a 3-part explanation in October with examples from recent works; see Principles of Animation for Motionographers – Part 3 of 3. Also, in the recent Lynda.com training series After Effects CS4 Beyond the Basics, Chad Perkins explores techniques for animation according to several of these principles.