Are you talking about the faux 3D film strip?
If you want that to move realistically, the easiest thing would be to model the shape in a 3D app like Blender (free) and then animate the texture. You could even do it all in C4D Lite that comes with After Effects but adding a movie as a texture would be more complicated.
The workflow you describe of animating the rectangles requires more artistic ability. You only have to create a long enough comp to get the rectangles to move to the next position. Think of it like a walk cycle. If you take ten frames for the rectangles to move to the same position as the rectangle ahead of it, you can nest that comp in a longer composition, apply Time Remapping, and add a loopOut() expression to the nested comp. You go to the last keyframe, move back one frame, set a new keyframe, then delete the last one, and you'll have a perfect loop that you can run as long as you want.
You could also render your ten frame animation to a visually lossless production format, import the footage, and then tell AE to loop it as many times as needed. Only render the first nine frames if you render it, so you get a seamless transition between the first and last.
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I hope this helps.
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