I eat pretty well and take pretty good care of myself. But it's taken quite a while to get here - before 2006, I had a pretty standard American diet. Lots of pizza, junk food, fast food, liquor, soda, sweets, etc. I smoked cigarettes, cigars, sheesha, and other kinds of tobacco.
Since then I've refined my diet and I eat pretty well. I have more energy, feel better, look better, and God willing, I'll live a lot longer as a result. It's a gradual process though, and I'm still improving. There's a few things I use to do it:
First, I'm all about incremental improvement - I think trying to crash change your diet is unlikely to work unless you have immense amounts of willpower and self-discipline. If you do have these Herculean amounts of will and discipline, you know who you are and don't need my advice. If you're more mortal, then you'll want to pick one or two things to be refining in your diet at a time.
Second, there's two ways I quit food or habits I don't like - "hard quitting" (cold turkey) and "soft quitting" (gradually reduce my consumption and eventually eliminate it). I pick which of these routes to go based on how convenient it is to quit something outright and if there's any detox process. If there's detox (like there was with nicotine), I think it's better to just get it over with once instead of constantly feeling deprived as your body re-adjusts to its new biochemical levels. The most successful method for quitting smoking is cold turkey, isn't it? Something like 80% of successful attempts to quit smoking are cold turkey? I don't have the statistics onhand, but that's the general idea. Quitting something like sugar, bad oils, or excess salt might be easier to do incrementally, since you need to replace the consumption with something else.
Which brings us to third point - I actively introduce new good behaviors before and during the time I quit something. Now, I don't know if the following is a good strategy, but it's what I did - when I started cutting down the sweets I ate, I increased my consumption of the kinds of salty foods I already ate: Chips, french fries, nuts, etc. Later I cut the salt content back. I don't know if that's a good habit, but it's worked okay for me. I also try to actively introduce fruits and vegetables before I quit something - it's hard to go from no fiber food that's highly processed to stimulate you immediately to fruits and vegetables. Fruit tastes bland compared to ice cream. So I introduce fruits and vegetables first, get comfortable with them, then increase my consumption of them as I decrease or eliminate bad consumption.
One of the largest productivity gains I ever got was going to a work computer / play computer setup.
I'd read about Paul Graham implementing it, and a few other people I respect. So when I switched over to Mac, I kept my old Toshiba for surfing the net and otherwise screwing around, and would only work or do work-like things on the Mac.
It worked well.
Not only was I more productive, I was happier. And I mean, really truly happier across the board. In the moment-by-moment, I was either fully disengaged and feeling good about it after working or when deciding to take a break. And when I was working, I focused. When I got to a hard problem, I kept going and pushed through it. It produced both more calm and more triumph. It was a good thing.
But then... eventually, things start to crack. And once they do, it's hard to get the magic up.