Do something nice for someone. If you see someone drop a stack of books, help pick them up. If you see someone trying to get something they paid for out of a vending machine (and you an afford it), buy one for yourself and let them keep the first. Drop an extra-big tip on a waiter or cabbie or delivery person who seems to be having a tough day. Or a busker who plays something you really enjoy, if you're so inclined.<p>It is absolutely shocking just how much the most trivial-seeming acts, when properly applied, can brighten a person's day. It is almost as shocking just how much seeing this effect, and knowing you're responsible for it, can brighten your own. There is no quicker pick-me-up, especially if you yourself are feeling down.