First, make sure you are powering the servo properly. Do not use the 5V Arduino output, you can destroy the Arduino that way.
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.
Torque specifications for the SM-S2309S at 4.8V: 13.90 oz-in (1.00 kg-cm), at 6V: 16.70 oz-in (1.20 kg-cm).