ALWAYS bring your blood sugar meter (and continuous glucose monitor if you use one).
Check your blood sugar before driving.
Keep the car stocked with fast acting carbs to treat low blood sugars (4-5 glucose tabs, 6-8 oz juice or regular soda)
Wear a medical ID (these are available for purchase at http://www.americanmedical-id.com/ if you do not already have one and should specify that you have diabetes).
Carry snacks to prevent low blood sugars (granola bars, fruit, crackers, etc).
If you are low, pull over as soon as safely possible.
Whenever possible, drive with a passenger who knows that you have diabetes and can recognize the symptoms of a low blood sugar.