Hi there Arthur here since last posting Slarti and I have been asked to put up or shutup by the White Mice. Now infinite may be stretching it but we quickly produced a wall 192 long and demanded an apology. The Vogon fleet were turned back I am glad to say.
The module we post now is critical and the basis of it all. It will raise and lower a stack of three pistons. Having tried various things found it was none too easy so we turned to the web. We found this http://minecraft.wonderhowto.com/how-to/stack-three-pistons-minecraft-and-become-king-world-0138126/ which explains how to do same.
So you need an ABBA switch, a vertical one in this case and a falling edge trigger. Watch the video by Austin Sirkin he explains clearly why. Essentially pistons are not that sticky!
Next problem was getting a configuration that allows extension. That done we arrive a the solution which is that we can continue to add piston stacks forever. BTW I use gravel or sand to push up and down.