From what I've learned:
1) Give them weight. This is done by having a rise and fall of the torso/head, and by having reason for it (fall when the leg is dropping, rise when the leg straightens under them).
2) Quick on the pass, slow on the linger. Use very few frames for fast movement, like a leg going forward, and more frames for a slower one, like a leg tilting as it lands or is about to be picked up.
3) Even scroll rate. Most of us don't walk in spurts, so feet should (when on the ground) travel at a relatively stable rate.
4) Tails and extremeties drag. Movement can always be broken down into the primary limb (leg, thigh, arm) and the extremities (lower leg, hands, feet, hair). These extremities always lag slightly in their movement. So if a man with long hair is moving left, then siddenly turns right his hair will turn right the *following* frame. If a man is waving his arms limply, the hands will turn 1 frame after the arms have.
5) Force via pointing. The positioning of details can have a great impact on your movement. For example, if your hand is limp and dragged slightly as an arm moves, the force is in the arm. If the fist is hard and pointing in the same direction of the movement, it's a fist leading a punch.
6) Roll up and down. Unless you're doing odd footwork (like tippy toes or something goofy) the foot should land on the ball of its heel and before it is pulled back up, it should roll onto its toes.
7) Center your weight. If the legs shift foreward, the body has to compensate by shifting backwards. Vice versa.
8 ) Whenever possible, don't reverse frames. Hands, feet, legs and arms don't move the same way outward as they do back inward. Unless you're working on something really simple, like a 3-frame reversed walk, the hands and details should reinforce the direction of movement by their slight lag. Without this, the animation will feel false.
9) Equal frame counts. You must have equal frame counts per step, with very similar positions per frame. Deviation from this will imply a limp. Because of this it's most natural of course to have walk cycles with even numbers of frames.
10) 1 or more frames with no feet on the ground makes a run or a skip. No frames entirely in the air make a walk.
11) Don't line up those limbs. Perspective is generally forced on a walk cycle so that the arms and legs hit different points when moving forward and backward. If you have your arms hit the exact same spot when they go back or forward, it'll look like 1 action happening twice.
12) If it's a sidescroller game, the feet can be lined up vertically. Any perspective and the back foot should be slightly higher up.
13) It's ok to shrink. Even though the actual perspective would keep the limbs closest to the camera and furthest the same size, shaving a few pixels off the back limbs can do wonders to differentiate the two.
14) Don't strobe. Whenevr possible, try to overlap pieces during small movement. If there's no overlap in an animation, it will look like a strobe light and won't read properly.
15) If you 3/4 the character (ie, you can see more than half his belly and his face) then you should shift the near limbs back to adjust for this. The close limbs will swing more left than the back ones, but the back ones will swing further right. However if you have a near-perfect sideview, the near limbs should swing further in both directions.
16) The leg straightens during the course of, and toward the end of the swing forward. Don't make it entirely straight too early, but also don't wait too late (such as when the leg's all the way forward).
17) Express impact where there should be some. Little things like a foot slightly wider on the impact frame, or a little bend to the knee when the foot touches down really sell the gravity of the foot hitting floor and supporting the shifting body weight.
That's about all I can think of now, although I'm sure there's more.
- Adam
