Write it down, record it in a spread sheet, save it on a web site... Late model Prius will track cumulative MPG, until you press the reset button, on purpose or by mistake, or disconnect the 12V battery. I always write my odometer reading, my screen MPG, and my trip reading (to get the .1 mile) on my gas purchase receipt and toss it in the glove box. At the end of the year I can tally miles, gallons, cost, and MPG.
CleanMPG or GreenHybrids have places on their sites where you can input all your data and it will do the calculations for you.
Congrats on the new green car. As mentioned on the above posts, you can simply put it in paper or in an Excel file - whatever is easier for you. I love using greenhybrid.com and enter my miles and fuel used in their database to give me an average data.
