Starting a new diet and exercise plan is easy; sticking with it is tough.
One of the big things that makes it so tough is the fact that those fast results from the first and possibly second week are going to slow down. This will happen to everyone and if you know this going in, it can help you maintain your focus and stick to your game plan when times get tough.
You need to go in with realistic expectations if you want to succeed.
In the beginning, your body is going to drop a lot of water weight and a few pounds of fat. While it is nice to see those first 3, 5, or even 10 pounds fall off those first few weeks, it can suddenly get discouraging when you only lose 1 pound the following week or worse when you go up a pound or two.
Your body is designed to survive when food is scarce. It's not just going to burn through all its fat stores in a week so that it can die of starvation the next week. When your body thinks food has suddenly become scarce it starts rationing (lowering metabolism). It does everything it can to keep from losing its fat reserves too quickly, but as long as you keep doing the right things, your body has no choice but to release stored fat to survive.
I have had many clients who stuck with their diet and exercise plan despite weeks (sometimes months) of frustration as their weight stayed maddeningly the same. They kept asking me why it wasn't working and I assured them that as long as they weren't cheating behind the scenes, it was working just fine. All those that truly stuck to the game plan would eventually come back to me and excitedly tell me about another sudden drop on the scale. You're body tries it's best to hold on to it's precious fat stores but it just can't help it if you consistently run a calorie deficit. The same water weight that pleasantly drops off in the beginning often holds on later in the diet giving you false plateaus to frustrate you. Often times, people are actually gaining muscle while shedding fat which keeps the scale locked on the same number even though they are progressing quite nicely.
Don't let that little number on the scale derail your motivation. Just be consistent, stick with your plan, and know that even after a quick start, 1 pound a week (or even a temporary plateau) is not failure.