Put those piano lessons you took as a kid to good use to make a little money on the side, or make a part-time business. You may not be playing in Carnegie Hall, but your piano teacher would be proud to hear you tickling the ivories at these money-making gigs!
Weddings are the perfect place to make some cash as a pianist. First, you need to put together a nice collection of wedding pieces (Mendelssohn's Wedding March, Pachelbel's Canon, etc.) and memorize them, if possible. Then, advertise. Put ads in the local paper, and leave business cards at wedding halls. Perform for free at a relative's wedding, then hand out business cards at the reception.
Holiday parties are another venue where you can find work as a pianist. Again, put together an appropriate set of music that you can play well, then advertise a month or two before the holiday season. Make sure you have a nice variety of pieces--slow, sentimental songs mixed with upbeat, jazzy numbers.
Religious services are also a good place to start making money playing the piano. Don't expect to make a lot of money playing in church services, but there are many congregations in need of a good pianist. Check the classifieds for openings, and be prepared to practice specific pieces given to you by the chorister.
Finally, the most obvious way to make money as a pianist is as a piano teacher. You don't need to have a Ph.D in music to teach--all you need is a set of books to work from (Alfred's, Bastien, and Schaum are all good). Become familiar with the lesson books and develop lesson plans, then advertise. Flyers at local schools, ads in the newspaper, and signs all work well. Make sure you don't under-charge--call around to find out the going rate, and don't be afraid to ask a competitive tuition.