Stay inside the vehicle. Hail falls at fast speeds, and it can cause injury to those in its path.
Stop driving and pull to a safe place so hail doesn't break the windshield or any windows — driving compounds hail's impact with your car. Stop under an overpass, and don't forget to pull out of traffic lanes and onto a shoulder. Avoid ditches due to possible high-rising water.
Keep your car angled so the hail is hitting the front of your car. Windshields are reinforced to withstand forward driving and pelting objects. Side windows and backglass are not, so they're much more susceptible to breakage.
Lie down, if possible, and keep your back to the windows. If you have a blanket, cover yourself with it to prevent possible debris from hitting you.