One thing that I've noticed over and over again in life, is that decisions get stacked one on top of another. If you make good decisions early in the day, you tend to continue to make good decisions throughout the day. This can relate to many things. If there's that one task at work that drives you nuts, getting it out of the way early in the day will make the rest of the day easy in comparison.
On the reverse, bad decisions can easily breed more bad decisions. I've had days when I've been running late, so I grab a sugary gas station cappuccino and a do-nut for breakfast. Next thing you know I'm eating hot dogs and french fries for lunch and choking down 3 cups of diet soda. The attitude is always, "well, my day is shot anyway." I might not consciously do this, but it happens. By the end of the day, I'm feeling groggy, and craving sugar, fat, and carbs. Out comes "depressed John" who orders up a pizza and a 2-liter of coke.
What can you take from this? First, try to start off your day with a good decision. Think of the first hour of your day as the foundation for the rest of your day. Get in that first healthy breakfast, or knockout a workout first thing. Then your decision making chain is starting to get a few good links in it.
If you should falter, don't sweat it. I just had some Mike and Ike's not 20 minutes ago. A co-worker handed me some and I took a few. However, I'm not going to let that affect my decisions on dinner and later. If you do happen to break your "good decision chain," take a moment, realize you did it, and recommit.