How to prevent recurrence of anal fistula?

Depends: On the cause of the fistula in the first place. Maintain a consistent bowel pattern and avoiding constipation or diarrhea is the best you can do. If the fistula is relate to crohn's disease, keeping your disease in remission by taking your maintenance meds without fail, is the best you can do.

Surgery: Once an anal fistula is established, it is very likely to permanently heal without surgery. There are many surgical options.