NXC Saturation algorithm for limiting motors?
By "smoother limit", if you mean "when it gets closer to the limit, it will softly land there by slowing down" then:
I recommend using PID control where the response (how fast the motor is moving to the limit) is controlled proportional to the error (how far away from the limit you are).

Wikipedia has a good intro to PID control: http://en.wikipedia.org/wiki/PID_controller