How do we prevent pml?

Assuming you have ms. Assuming you mean progressive multifocal leukoencephalopathy, and assuming you are on tysabri, (natalizumab) your dr. Now has the availability of serum titters to see if you are at risk. If you are, the only means of prevention is to follow titters.
Resource. No guarantees, but staying in good shape overall with a strong immune system may help, see: http://goo.Gl/hnwtc.