Plan, plan, plan.
Setting aside an hour one day a week to do things like chop veggies, throw some chicken breasts on a grill or george foreman grill, make a pot of veggie soup, etc., means that later in the week when you're busy, you'll have food around.
I sit down every Sunday evening and make a quick meal plan - it's not obessive, it just gives me time to think over the coming week and consider when I'll have time to cook and when I won't. Knowing in advance that I won't have time to fix dinner on Thursday and so should have something in the fridge already makes it easy to make healthy choices.