If you have long hair, sweep it up into a ponytail. Put on some earrings and a little lipstick.
If you are a man, a clean shirt and quick brushing of the hair does wonders.
Everyone should at least look clean, unwrinkled and have decent shoes on their feet.
Add a hat or ballcap if you don't have time to deal with your hair. It at least looks like you are put together.