So we got the battery sorted out, people were concerned you just might be using a "PP4" battery. 
OK, so you are not that silly!
If you are not using PWM, a 0.1µF ceramic capacitor soldered directly across the motor terminals. YOu will notice this as standard in radio controlled toys. (Serious models use ESC.)
If you are using PWM, you can still do this, but will need to provide an inductor in series with one or each of the motor terminals - I will "bow out" at this point, of explaining how you should calculate the value.