I've had good results with a pretty average balanced "diet," and sticking with the workouts. It's probably CrossFit heresy to say so, but I don't even use the Zone diet (although I have nothing against it).
I have come to realize that I used to eat too much, though. I've found that not approaching meals with the "I can finish everything on this plate" attitude was a good start. I realized that I actually get full eating about half of some meals that I used to eat all of--huge plates of Pad Thai, for example. Great way to cut the calories. I also found that nuts are nice and filling for snacking, although they do pack some fat in them.
A lot of supplements are just cr*p anyway (can you say "crap" here? Honest question!). Even the effectiveness of multivitamins has inconclusive support, with one study leaning one way, and one study leaning the other. There's no disagreement about the effectiveness of getting nutrients from food, though, as FG noted above.