The Nike+ Sensor is designed specifically for Nike+ shoes. The Nike+ Sensor works best when placed in the pocket of the left shoe, under the sockliner. By placing the sensor anywhere other than this pocket, accuracy, reliability and wireless performance could be affected and lead to inconsistent or unreliable performance.
If you do not use Nike+ Ready shoes, keep in mind that the sensor needs to be flat, logo-side up, length pointing forward (it should not sit side-to-side), stable and secure to obtain accurate information about your run. And since sensor accuracy out-of-the-box is diminished when not using Nike+ ready shoes, calibrate your sensor to your running and walking stride.