Few situations are less enjoyable or more dangerous than when freezing rain decides to freeze on your windshield. There are several things a driver can do to assure this doesn't happen:
The first solution is to treat the windshield with Rain-X or some other rain repellant. There are other brands, but Rain-X is most universally available. With the vehicle in a warm, dry place, apply the Rain-X to the clean windshield following the directions on the bottle. Make sure to polish the residue film off with a clean dry cloth as per the directions. Now the water often won't stay on the windshield as it won't stick to the super-slippery surface even if it starts to freeze. For light freezing rain, this often solves the problem entirely.
Next, make sure you have replaced your wiper blades with good all-weather blades or silicone-coated blades. The all-weather blades are designed to drain the water away from the blade edge to avoid freezing. The silicone blades work much like the Rain-X in providing a non-stick surface for freezing rain to stick to. Even the best wiper blades will cost less than thirty dollars for the pair and can be installed yourself. If unsure of the process, have your local garage install them and have them show you the process. It is a good idea to always have an extra pair of wipers stashed in the trunk as heavily iced wipers have a tendency to break off, leaving you "wiperless". In a pinch, a passenger side wiper can be removed and placed on the driver's side as that is the side that usually gives you the most trouble due to water thrown up by passing vehicles. If you need to switch the blades, make sure to cover the tip of the empty wiper arm with a copious amount of duct tape or take the entire arm off: the empty arm will continue to make contact with the windshield on every pass and will scratch it badly. Heated wiper blades are the all-around best alternative. If your vehicle is equipped with heated blades, very good. If not, they can be retrofitted but are rather expensive.
Always fill your windshield washer reservoir with windshield de-icing washer fluid. Plain water or regular windshield washer fluid will actually make your problem worse as it freezes on the already frozen wipers and windshield. A spare jug carried in the trunk will be handy for emergency refills.
On the inside of the vehicle, your job is to make sure heat from the defrosters can reach the window. Keep your defroster vents clear by removing any loose items from the dash. You must allow space for defroster heat to circulate to all sections of the windshield. Keep the defroster heat on high blower for best coverage. In some vehicles, it helps to lower the sun visors to keep heat circulating against the windshield instead of defusing into the vehicle. If your heater fails you, one option that sometimes works is to turn the defrosters off entirely, allowing the glass to approach the temperature of the outside air. As windshield freeze most often occurs when snow melts on a warm windshield then refreezes, keeping a cold windshield can help avoid the problem. If the vehicle is equipped with a vent wing-window, opening it and directing it onto the inside of the windshield will help cool it. Hopefully, you have your cold weather gear to put on as this wont be very comfortable for you.
One critical winter item to have handy in your vehicle is spray windshield de-icer. If in spite of your efforts, you build up ice on your wipers or windshield, pull off on a highway ramp, rest area or parking lot, get out and spray the ice with the de-icer. Then sit until it does its job. While you're stopped is a good time to clear your head and tail lights: if the windshield is freezing, often these critical lights are also. You want to be very visible to others on the road that are also fighting poor visibility and may not be as prepared as you are.
If in spite of your preparation and efforts you simply cannot keep your windshield clear, stop. Pull over and wait it out-call home or to your destination and let them know you will be late. Usually icing conditions don't last very long as they are often a transition phase to snow. Better safe than sorry!