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Using an expression is an easy way to prevent having to create many keyframes manually to create a looping animation in After Effects.  This free download includes a PDF tip and After Effects project file that shows you how to use the expression loopOut to create a looping animation using only two keyframes.

You can also read the tip at our blog:
http://blog.aeprojects.com/2006/08/24/simple-keyframe-looping/