I read "The 150 Healthiest Foods on Earth" (Jonny Bowden) recently and took from it this extremely condensed diet advice: "Avoid grains and industrial processing." Grains have near-zero nutrition, and industrial processing is the source of a lot of questionable practices.
On this diet, which I've followed just a few weeks, I cheat all the time, but I feel and perform better in ways that I can quantify at the gym: more strength, better endurance. Noticeable improvements every time I've gone, so far.