Anyone who writes novelty slogans or greeting cards should consider joining up with Slogan Slingers. It's a web site that tries to pair businesses that need slogans or tag lines with writers of slogans and tag lines -- really a great idea. It's free to sign up, and if your slogan is chosen you could earn from $200 to $999 (minus a 10% administrative fee). Once you sign up you'll be able to see and enter the current slogan contests, and the site is good about getting the contest holder to provide as many details and specifics as possible to help you write the best slogan.
(Thanks to CyberForensicWriter for the tip!)