When animating, remember to model animations after yourself. Animations must be natural. While idle and breathing, remember that to inhale at rest takes longer than to exhale. Animations must be slight, and the entire body cannot remain static. Hold your index finger aloft and watch. There will be slight shaking, unavoidable movement.
For attacks, mind the weapon and the user. A realistic sword can be as tall as a man, and comparably thick, yet still comfortable to use, and easy to swing, while a regular sledgehammer requires MUCH less dexterity. effective attacks with the sledgehammer require good use of momentum. The body is a spring, preparing to launch the weapon, still in the hands.
To jump or leap, remember the intent, and mind the Y axis. A standing body (I) should tilt forward before leaping(/), extending the legs while in the air(\), with the knees and torso buckling slightly(/), as the body recoils, before returning to a standing position(I).
Death animations should be thought out before-hand. Use the World Editor to find a death sound that matches the intended voice of your model. Match its duration, come up with a pose for the corpse to assume. If there is a sound for the corpse hitting the dirt, be sure to match the animation to the sound.
Remember, a normal human body is quite heavy. Upon death, the strength to keep it on its feet is nearly gone. A falling body must reflect this.