Kerosene, or gas. RainX helps a little on the windshield, as does a good quality wax to protect the paint. Use a WORN OUT kitchen pot scrubbing pad (like the Scotch Bites), as they will abrade the sap without removing the paint. It's much more soluble when it's warm or hot. Some people use a little Kerosene in water to clean thier windshields, and I'd venture a guess that this practice would help stop the sap from sticking to it, in the first place.