If you use the Accelerometer to control 'speed' you might experiment with these properties to change the behavior of the ball;:
MinimumInterval
Specifies the minimum interval required between back-to-back Shaking events, in milliseconds. Once the phone starts being shaken, all further Shaking events will be ignored until the interval has elapsed.
Sensitivity
Specifies the sensitivity of the accelerometer. Valid values are: 1 (weak), 2 (moderate), and 3 (strong).
An alternative is to NOT use the Accelerometer. Here is an example discussion Need help for making a maze game!