It happens to us all at some time or another. You're minding your own business in the outdoors when you feel something warm land on your head, or you spot something white on your clothes. A bird has kindly relieved itself on you! Firstly, stay calm (don't panic). Flapping and flailing your arms will only cause the bird poo to dribble and splatter, further covering you and any companions you may have. If no one saw it happen, pretend it isn't there. Chances are that no one will comment for fear of looking stupid when you tell them it is a fashion accessory.
If people do notice then wipe it off. Do not use your hand, do not use a passer-by's hair. Use a handkerchief or a leaf. Do not put the handkerchief/leaf back in your pocket. You can also use anything with a rigid edge to scrape it off. This will only be a temporary measure, because it will leave the stain, but it will work on wet or dry poo. Act cool. Tell people it is good luck to get crapped on by a bird. Make them jealous. Find water and clean it up as best you can. In extreme circumstances go straight home and wash your clothes.
Bird poo is known as guano, and can sometimes carry diseases and fungi, although it's unlikely that you'll catch anything from a stray plop on the head. If you find yourself having to clean lots of it from surfaces, mist it with water and wear a face mask to avoid inhaling anything nasty. Watch out for flu-like symptoms.
It can destroy the paint on cars because it's very acidic. Try not to park under trees, and clean it off when you see it. It's particularly potent on sunny days and dark colours. Remember that birds swallow grit and small pebbles to aid digestion, so soak it in water and lift it off, rather than rubbing it off, to avoid scratching the paintwork.