A few good ways to remove tree sap from a car without damaging your car paint is to use either some Nail Polish Remover or some Mineral Spirits.
Apply either some nail polish remover or Mineral Spirits to a sponge, washcloth or a cotton ball.
Then wipe the nail polish remover or mineral spirits onto the sap and let it sit for a few minutes and then scrub it off easily.
You can also use some WD-40 or lighting fluid to remove the tree sap safely.
After removing the tree sap from your car I would recommend using a tarp or drop cloth on the car when under a tree to prevent tree sap from getting back onto the car.
Some Purple Power Cleaner and degreaser can also safely clean the tree sap and other residue off your car without doing any damage to the car paint.