See this Sample Copyright Assignment form.
You could also sign over only limited rights, such as a once-off reproduction or the right to a very specific use (for example on a T-shirt or card). Be very specific about the rights you are signing over and your payment. Consider whether you want to insert a clause about the rights reverting to you after a certain amount of time or if the company goes out of business.
Go to Full Artist's Copyright FAQ.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.

