I think that the holder for the Nike+ sensor is a bad design. It is very difficult to open and I am always afraid that I would break it.Thus, it is also very inconvenient for me to attach it onto my shoelace. The slider to open the case to put the sensor is also very stiff, and it is also very hard to open or close.
My suggestion would be to make the case slider easier and smoother to open, and make the clip less stiff. This would enable me to conveniently put it onto my shoelace.