How do I avoid developing leukemia?

Nothing. I do not know of anything that can prevent leukemia. A study at mayo clinic by dr. Shanafelt was looking at treating early stage cll with green tea extract. Further studies will likely go forward. Some risk factors, such as prior chemotherapy for other cancers can increase the risk of leukemia, but that is likely not avoidable. I am unaware of any diet or exercise intervention.