Slightly rewarded (slightly punished)
For most of us, it's not the big traps that mess us up, it's the little ones.
Every time I break stride and distract myself by checking my email (a hundred times in a bad day), I get a small reward. I get the satisfaction of starting and finishing a project, on time and for free.
For a lot of people, every time they drink a Coke instead of a glass of water, they get a small punishment in exchange for their treat. One Coke never hurt anyone, but a hundred of them make you fat.
One way to change behavior is to keep track of how often these little events occur, because seeing them lined up on the windowsill might be enough to change your mind. The other way is to make those events louder. I'm pretty sure that if I got an electric shock every time I stopped to check my email, I'd only do it daily…