You could try covering the sensor with some foam like the one in this article: http://letsmakerobots.com/node/1276
You might also try critically looking at your code. Maybe the variable you assigned to the distance cannot store a number high enough, so it is overflowing?
How far is your sensor from the ground? If your sensor is near the ground, its signal might be bouncing off the ground.
I had similar problems with mine, and I was lucky enough to have an extra PING sensor lying around. So I kept all the programming and circuitry the same and simply replaced the original PING sensor with another sensor that I knew worked perfectly. It turned out that I had a bad sensor. Hopefully, it's not your sensor. Good luck to you!