The Hardware Hacking guys have posted this brief tutorial with tips on improving the accuracy of ADC reading with the Arduino. They endeavor to do this by measuring the internal 1.1V reference voltage (it is available for ATMega 168 and ATMega 328 only) and, then, by normalizing the ADC.
Via the Arduino Blog.
They make the common error of dividing by 1023 instead of the correct 1024 to calculate the voltage.
You won’t get anything accurate when you do it like this.