Sonja Lyubomirsky and her team studied this in 2004. She had a group of test subjects put quarters into strangers' parking meters. By measuring the test subjects' happiness levels before and after the quartering (and comparing them to those of a control group, of course), she found that performing kind acts really does make people happier. Some call it a "giver's high." It works even if the recipient of the kind act isn't aware that you did anything (or that you even exist)!
Interestingly, Sonja's tests show that the best way to perform kind acts is to do a bunch of them all in a single day, instead of spreading them out over a week. The reason may be that pushing yourself to do one act every day becomes too routine, while doing around five all at once is more spontaneous, which keeps things fresh and makes you more likely to be kind in the future.
So give kindness a try. It can be as simple as putting quarters in a stranger's parking meter!
(For more details on this experiment and many others, check out Sonja's book, The How of Happiness.)