jremington:
Use a separate servo power supply, like the 4XAA pack mentioned above, and connect the grounds. If the servo still does not have enough power, buy a higher torque servo.
And up the supply current to match the higher torque servo - they will need much more current.
For some reason the specifications for RC servos tend to omit the current requirement which is
very annoying as peak current draw is hard to measure with just a multimeter.