Vigilantly looking out for deer is the first step. Deer are more common in the roads in late fall and winter. They are also nocturnal, so be sure to keep an eye out at night. Where there is one deer, there will most likely be others.
Slow down when you see a deer. If you can avoid hitting a deer, absolutely do so. Some people say that it's best to speed up to avoid a deer possibly going through your windshield and injuring or killing you and other passengers. Do not do this! If you can slow down or stop to avoid the deer, do that first.