It'll get better.
1)It's cold,
2)You're not even broken in yet,
3)Have you adjusted the tire pressure (most recommend 42f/40r or even 44f/42r),
4)Have you checked that your oil is not overfilled even slightly above the full mark?,
5)Most recommend having the alignment checked,
6)Winter gas is in use most places,
7)Don't accellerate slowly--accellerate fairly briskly--the ICE is most efficient at around 4000RPM (best fuel efficiency to power ratio), get up to speed, then set the cruise control.
8)anticipate stops and begin braking early.
9)The EPA numbers are artificially elevated , esp in a car like the Prius (check the Yahoo forums for details).
10)Remember that short trips from cold starts are the least efficient--your first 5 min will always suck, if your trips are only 5-10 miles then you're just getting warmed up when you have to stop and you're just not going to get into the Hybrid's most efficient zone.
Finally, I took all kinds of heat over on the yahoo board for mentioning that I was disappointed in not seeing 55mpg combined (I'm seeing about 45mpg at 1800 miles). Many people mentioned the things above. Those EPA numbers can and will be achieved, occassionally by some drivers and never by others. There are lots of variable involved. Give it time, watch that Energy screen to get an idea what's going on, pay attention to your driving style and You'll start seeing those numbers go up (at least after winter is over). Also, remember that you calculate milage may differ a bit from what the computer claims you're getting.
--evan