I have an Arduino microcontroller. I wanted to control a few servos for an example project but I soon found out the Arduino can't give it enough power. I don't understand how to power something externally but still use the Arduino to control it.
I think you may not fully understand how the servo works. Most servos have a power, ground, and PWM line. The PWM line will pull very little power. This line is what the microcontroller will actually be controlling. The Power and Ground lines are where the servo actually pulls its power from.
The power line can be connected to anything as long as the ground lines are at the same potential as the Arduino. If you are using an Arduino board, the voltage regulator that is on the board is limiting your power to the servo and not the microcontroller itself. If you are using an Arduino in an IC form (ie: breadboard) then you will need to make sure what ever you are using as your voltage source (power supply, battery, voltage regulator...) is at the right Voltage level and can handle the current of the servo.
There are several ways to do this. I suggest you read the following excellent tutorial: http://itp.nyu.edu/physcomp/Tutorials/HighCurrentLoads
As @Kellenjb says, power your servo directly from your main battery/plugpack. Connect servo ground to same ground as arduino, servo power to your power supply (or a separate regulator if your power supply voltage is too high for the servo) and connect the servo's control line to the arduino.
If you are powering the arduino over USB, there's only enough power available on the USB interface to drive a small servo, so you should consider a separate power supply.
You might want to look into the Motor Shield from Adafruit (http://www.ladyada.net/make/mshield/index.html ). It gives you an easy way to add an external power source.