Let's start at the beginning (a very good place to start). When you boil a walk down to its essence, what you are doing is falling and catching yourself. Gravity is trying to pull you into the ground, and you're fighting it for a brief moment (while your foot is up) before giving in as you move forward. This is a VERY important concept to grasp. The biggest issue that crops up in a “bad walk” is a lack of weight. Remember: It's a controlled fall. Your character is fighting constant gravity and recovering with each step. Use that knowledge when you're planning your poses. Feel that weight.