Accurately measuring temperature with Arduino
I think you should consider using a digital temperature sensor like DS18B20/ DS18S20 because it does not depend on the accuracy of your ATmega ADC to measure an anaolg signal, it uses 1-wire digital protocol to report temperature. 

Refer to the following tutorials
http://playground.arduino.cc/Learning/OneWire
http://www.hobbytronics.co.uk/ds18b20-arduino