How can you prevent leukemia?

You can't. Unless you are being exposed to extremely high levels of radiation or you have had chemotherapy or radiation therapy for another cancer, most leukemia is not preventable.
Not much... Many times we don't even know what causes leukemia so we can't tell exactly how to prevent it. The process is probably multifactorial and has a strong genetic component. In a small subset of cases exposure to various toxins like paint fumes, or to radiation, can cause it so avoiding those is a minor way to prevent it. But we can't do much about the genetic predisposition...