How does that thing actually work?
It works just like a pot. When you stop pressing the wiper is disconnected so it floats. Therefore you have to put a high value pull up or pull down resistor on the input to define the value un-pushed. In the UK you can get them here:-
http://www.totalrobots.com/sensors-flexforce-c-71_72_77.html?main_page=index&cPath=71_72_77