What can I do to prevent or lessen the chance of avascular necrosis?

Options limited. Depends on the location and manner of fracture. Shoulder & hips at highest risk for avascular bone necrosis, so only real prevention is not getting injured to begin with. In some special cases you can take steps to reduce risk. Deep sea divers (due to the bends) and patients on high dose long term corticosteroids are more at risk. Talk to your dr.
The top ten things . That cause avn is alcohol, alcohol, alcohol, etc. The other minor causes include excessive steroid use, metabolic diseases, deep sea diving, trauma to the involved joint. Not a common diagnosis in general though.