Use diluted baby shampoo to get bird shit off your paint as soon as possible, even if you don't have time to wash your car right away. It's gentle enough for babies, and it breaks that nasty stuff up quickly, so you can wipe it away. It's also especially effective if you have birds in your area that have been eating alarmingly bright berries.
Let the baby shampoo stay on the car too long -- especially in the sun. We don't want you running the risk of having areas of faded paint on your ride.