How does craigslist retain search query results?

When you search for a keyword on craigslist, a list of posts is
returned. When you click on an individual post in that list, the full
post is displayed. Then, when you press the back button on your
browser, the results list is displayed again, without the server having
to query again for the list of keywords. How can I get the server to
remember search results without having to requery the database when the
back button is pressed?

Thanks!

That is not done by the server at all. That is completely a function
of the client's browser history.

In order for this to work properly the URL must be different so that a
new history item is created. There are other ways of handling this as
in DHTML History.