Good nutrition is very important during recovery. Your mind cannot truly heal without good food. Stay away from fad diets and avoid junk food--in general, it is good to avoid extreme changes during recovery.
As for preventing relapses, fellowship has worked for me for 50 days. First, I spent a week in detox with "kids" who were trying not to ruin their lives with heroin. They taught me my life was worth saving. We had nightly AA or NA meetings, and that was a good introduction because my expectations were not too high for my local groups when I got home. Meetings almost every night and a helpful sponsor have worked for the past seven weeks.