I've learned in my 30-something years in this life that planning is the key to eating healthy. Sometimes I slip (like just two weeks ago when I started a new job and did not plan my lunches), but I always go back to my planning ritual.
I pick a day of the week when I'm off, like Saturday, and that's also the day there's the farmers market so I can stock on fresh produce. For most people with normal jobs the planning and shopping day will be Sunday. Grocery shopping is the single most important thing when it comes to staying on track with a healthy eating plan.