If you have to replace your batteries....you might as well get better......most not all servos can run on 6 volts...this will make your servos faster and have more torque...If you don't want to go 6 volt that's fine get a battery with more capacity...battery capacity is measured in mAh which stands for milli Amp hour...a 600 mAh battery in a perfect word will put out 600 mA for 1 hour...so get a 1100 mAH battery to replace standard batteries and you have almost twice the capacity...
the other issue is a battery charger if you go 6 volt...the standard wall wart just won't get it done...you'll need a charger.....perferably one that you can charge nicads or nimh batteries
Good Flyin