I don’t have a consistent approach to this yet… I’ve been hunting around today reading articles on different techniques for styling visited links on a web page.

I agree with this comment on usability advantages of styling visited links differently from un-visited links, emphasis mine:

“Visited links not only help users avoid pages they don’t want to go to, they help users find pages they want to get back to. Styling the visited links in your site’s content will vastly improve your users’ experience.” [Source]

I think this point is often overlooked.

More articles I found useful:

• Mostly solid advice that has stood the test of time: http://www.designbyfire.com/deye_web/alertbox.htm
• Thoughts on using a background image next to a link to indicate that it has been visited: http://www.colly.com/comments/ticked_off_links_reloaded/
• Using symbols to indicate visited links, good points in the comments too: http://www.mikeindustries.com/blog/archive/2004/06/radical-links

I’m a big believer in using an underline as well as color to display links in general (with exceptions). I’m leaning toward using a washed-out version of the link color to display visited links in the main content for a web site, and possibly a background image or symbol to indicate visited links in a side bar or list.