If you can’t park your car in a garage or securely off the street for the night, your best defense is a fresh coat of wax. This will add a layer of protection against eggs, silly string and the like. It’s also good prep for the road salt, grit and other rigors of winter driving that are just around the corner.
If your vehicle does get egged, wash it off asap. As WTOP.com points out, “If eggs and other messes are left on the paint of an older vehicle for a period of time, they can cause permanent stains and even lower your vehicle's resale value.”