Meditation helps. When I take even 8 minutes a day to sit quietly and breath (I use the Timeless timer for iPhone), I find I have more willpower to avoid crappy food and trivial time-wasters.

Similarly, making sure you get daily exercise and spend some time in nature if you can. Apparently, according to “The Distracted Mind”, even just looking at pictures of nature can help your brain recharge, allowing you to be more focused.