Hold a comic book drive. Let local comic book stores know about your event and ask to post fliers in their store giving the information about the drive. You may also want to put an ad in the local papers about the drive.
Organize a party for the comic book drive. Provide food and entertainment for anyone who brings comic books to donate. Ask a few friends to dress up as comic book characters and work the event.
Post an online ad. Write up an ad for the Freecycle Network website. Ask people to send you their old comic books instead of throwing them out. You should pay for any shipping costs incurred by the sender.
Offer a trade. People are typically reluctant to part with something if they aren't getting anything in return. If you don't get enough comic books during the drive, offer up CDs, DVDS, or old video games to people who donate to you afterwards.