A new tutorial online at flashtuts+ shows how to use Adobe Flash to create a looping run cycle.  While the mechanics are Flash-centric, the theory he covers could be applied to any tool.

In this tutorial we’ll create a quad run cycle. The animation will be created using simple lines. Along the way we’ll cover a bit of work flow and animation theory, then near the end we’ll use Motion Tween and ActionScript to move the animal across the screen.

via Animate a Continuous Quad Run Cycle With Flash – Flashtuts+.