Hi, I'm now on day 6 and day 3 was definately my hardest! Lots of water as the lovely people above me have said - the other thing that kept me motivated was thinking about my first weigh in, and how proud I would be if I could tell my CDC that I had completed my first week cheat free (which I'm glad to say I have!). Another thing I found helped was making some ice lollies using water and the water flavoring, makes for a nice (and cheat free!) snack! So lots and lots of water, and keep focused on your first weight in!
Good luck, keep me updated on your progress!