If using a slide pot, you can change it's range mechanically ... think  
gear-shift in a car.

Alternatively, I saw a very nice (and quite reliable) "slide  
touch-switch" implementation using on of the enhanced PIC16F's at a  
microchip seminar recently.  I was quite impressed and tested it with  
a few sheets of paper between the contacts and my finger, which also  
worked beautifully.

If that's not an option, then how about using whatever mechanical  
switch mechanism you come up with, together with a belt plus 2 pulleys  
(in a loop) and mounting a quadrature encoder on one pulley.

Cheers,
-Neil.


Quoting dpharris@telus.net:
>
> Why not a slide potentiometer?  If you need indents, then perhaps   
> you could add
> them.  Likely to hold up better and easy to obtain, eg
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PP1045SD-ND
>
> David


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