Marshall Kirkpatrick at ReadWriteWeb lays it out:

Turn on Creative Commons Licensing

It’s easy to turn the default setting for new photos uploaded to Creative Commons Attribution (our favorite) by visiting the Privacy & Permissions tab in your account. Unfortunately there’s not clear, working links from Flickr to an explanation of the different licenses. Here they are on the Creative Commons site.

CC Attribution is a license that says other people can use it and change it, including in a commercial context, as long as they give you attribution as the creator. It greases the wheels for quick and easy media sharing. That’s good and it would be nice if more quality media was licensed this way. We keep a link to the Creative Commons by Attribution search on Flickr in our browser toolbar and use it frequently for photos in posts. Those could be your photos we and others are using!

(Emphasis added.)

Read the whole article for Marshall’s other helpful suggestions on how to make the most out of Flickr.