The brothers Heath are personal favorites of mine--their other big book, Made To Stick, is a great read as well. But I bring up Switch here because of it's relevance to building healthy diet and exercise routines.
This book is about habits. Why people form them, why they get stuck in them--and how to change them. The Heaths look at numerous case studies and applications from all over, but I think you can see how that sort of information would be useful to trying to curb the junk food or get into the gym.
I want to paraphrase two key concepts they discuss here, because they've stuck with me years after my first read:
1. Willpower is a consumable resource. Some people have a ton of it, some people have a little bit--but everyone has a tank that empties if you rely on it too much. Think about how this frames your diet and exercise in the context of your whole day. If you planned nothing else, going to they gym is easy. But if you spend all morning forcing yourself to do things you didn't want to (paying taxes, maybe), it becomes harder to choose the salad over the burger at lunch. And if you force yourself to do that, deciding to go to the gym after work vs. going home for a nap and a netflix binge suddenly becomes a hard call. Which brings us to:
2. Emotion, willpower, and environment (presented metaphorically as an elephant, it's rider, and the path they're walking on). Your rational willpower--a little dude riding a big, impulsive emotion-elephant--wants long term goals. Better health, six-pack abs, a healthy savings account, whatever. The elephant doesn't give a damn about any of it. The elephant sees a cookie. The elephant wants a cookie. The rider can force the elephant away form the cookie...but it's exhausting. He can't do that for every decision. The elephant is just a lot bigger. So how do you steer it long-term?
Mainly, you find ways to stop fighting and get the elephant on-board with the rider's goals. More carrot, less stick, as it were. The elephant is going to choose "cookie now" over "kale now," because it doesn't think long-term. But if you set up your diet to be "cookie now" vs. "steak dinner now," suddenly it's a different equation. Yeah, you want that burger...but you've got that really tasty chicken dish at home (elephant: "yum!" rider: "healthy!"). Going to the gym is hard, but seeing your friends is fun (elephant: "hello friends!" rider: "exercise is good!").
The other option is to control the environment--change the path so the elephant runs into positive situations and avoids negative ones. I find I struggle if I go home from work and try to go to the gym after I've settled in on the couch for a while...so I go straight from work as a matter of course. This has become so effective that sometimes I even show up on my rest days, say hi to everyone, and then leave. It's almost a problem.
If I go out for lunch, hungry, I know I have a habit of tacking on a soda or a cookie to my otherwise passably healthy order. So I pack a healthy lunch the night before, after dinner, while I (and the elephant) am already full.
That's really the tip of the iceberg here. Just...just read the book. It's worth it. Look, I found it helpful enough that I'm here, recommending it to you, and they aren't even paying me. This is unsolicited. They don't even know I exist (probably--I guess they could stalk me on facebook or something).
Read the book...and keep earning it.