It would seem that your big mistake is in your code.
You will never get a consistent reading from any A to D circuit. At the very best they +/- one significant bit. So the If statement condition will hardly ever be true. You need to add a range.
if ( abs(val - last_val) > range) {
Where range is about 8 or so.
Also note that of you want to use your finger then this will be supplying noise.
Note that you made the circuit from the Instructables site. Almost all projects on this site are written by people who don't know as much as they think they do. So unless you are good at spotting errors, never use this site to try to learn something new.
I did make a music instrument called a Spoonduino, which used a square of this material. This is a link to the story behind it along with a video of it in action.