the servos i know from TowerHobbies or Futaba they usually work with a pulse width modulation signal, that doesn't mean you need to make a pwm'd power source, it has a 3 way connector, a ground, a +5 volt and a control signal ... well, the only way to work is to get the center speed spulse width, that may be on the manufactors website ...
You need to make pulse's larger ou thicker than the center pulse width to speed up or down the servo ...
It's usually easy, but be carefull with the control signal way in the connector, excessive power may burn the digital interface inside the servo ...