The best method is to invest in a decent car cover. Second to that, remove the bird droppings as soon as possible. They will do much less damage to the paintwork if they don't sit on it for days at a time. Spray the droppings and wipe it off with a cloth, Meguiar's Quik Detailer (amzn.to/2S029MZ) is a good choice. Also, keep the car waxed. This will make cleaning bird droppings off easier. We generally recommend a polish like Poorboys White Diamond (amzn.to/2W4qYaA), followed by a sealant like Bilt Hamber Double Speed-Wax (amzn.to/2Wvdg59). Autoglym Rapid Aqua Wax Complete Kit is also a good option: amzn.to/2RS0rgA