Eat Organic. Most meats and other foods commonly consumed today have large doses of antibiotics and hormones with feminizing effects. Also avoid consumption of large doses of alcohol and definitely avoid marijuana (a known cause of gynecomastia).
Soy. Some foods and nutritional products have estrogenic properties which can cause gynecomastia in men. These include soy products, black cohosh, red clover, lavender and tea-tree oil. Alcohol and marijuana use can also cause gynecomastia.
No. Gynecomastia, enlargement of the male breast, is unrelated to food intake. Enlargement of the female breast (sometimes referred to as "macromastia") may be related to weight gain, since the female breast often has a large component of fat. In that case, the best way help to reduce breast size is to go on a weight-reduction diet.
Yes. If you consider beer a food :) too much alcohol consumption can trigger or increase gynecomastia. Some suggest that reducing meat or milk raised with hormones may also help. Reducing this meat is plausible to me (not the milk), but I haven't seen studies looking into this. Certain pesticides also have estrogenic effects. Decreasing these are no guarantee, but might tilt the odds of improving.