For cheap and reliable tuk-tuk service in Phnom Penh, call David (or Bunthoeun if you'd rather use his Cambodian name). David can provide one-off journeys or you can hire him for the whole day at US$20.
Forget taxi's, tuk-tuk's are the way to go. You see more and the heat is taken away by the cool wind that flows through the trailer. No need for a taxi, they cost too much and have no real advantage. Tuk-tuks can get through traffic almost as easily as a moped.
David is a good tour guide, tell him what you want to see and he will plan the best route for you, fitting in as much as possible in a short amount of time.