The easiest way to contribute is to donate. The costs of running the site are covered by the Common Lisp Foundation.
Click here to donate to common-lisp.net specifically. Thank you for your support!
If you would like to contribute by donating your time, please let us know (through a mail to this mailinglist, for example). There are currently a few projects that need attention, and we almost always can use a helping hand.
Two projects waiting for volunteers are
Also, if you have a cool idea for the site, feel free to let us know.