When you redesign a site, chances are that your structure may change and your URLs may change. If you don't plan ahead, you could throw away a lot of the search engine optimization you may have already achieved. When a search engine comes back to index your site again after a redesign, it will find that many (or all) of the old URLs no longer function and it will drop them and then reindex your new pages as new pages instead of as new versions of existing pages (which already have some ranking). Also, other sites may be linking to your site using old URLs (remember tip number three about external links?) and it would be a shame to throw that away. Of course, all this is not to mention the frustration your users may experience if they've bookmarked an old link or travel in to a 404 error page from a search result.

You can set up redirects manually using your .htaccess file or using a component or module for your  shiny new content management system (which would give you an administrative interface to manage these redirects -- for example, the "Path Redirect" module for Drupal).

There have been a number of high-profile cases recently where this has not happened (notably the White House site, which dumped the previous site with a lot of its URLs "down the memory hole" and the New York Times, which erased a lot of content, many URLs, and one journalist's career when it wrapped the IHT site into its own).

Recommendations for dealing with old URLs before your redesign is completed and launched:

1. See if there is an easy way to set up redirects for all URLs (e.g. if you are simply moving from a sub-directory to the root directory, you can redirect all traffic that comes to that sub-directory by removing the sub-directory from the URL)
2. Check your stats and see which pages/images get the most traffic -- definitely set up redirects for these, and ideally set up redirects that cover all pages from your previous site and point them to individual new locations.
3. Make sure that any internal links in content that has been ported over to the new site have been updated to the new URLs.
4. In some cases it makes sense to redirect any traffic coming to a certain directory to a specific page (e.g. when we redesigned our site, we first set up the redirects for the pages that were visited regularly. We had a number of less-trafficked static HTML pages in the portfolio directory and we simply set up a rule to forward any "left over" traffic to this directory which didn't get redirected by the earlier rules to go to our main portfolio page.) You want to avoid blanket redirects as much as possible but it can be useful in certain instances.

recommendation for after launch

1. Keep track of your error log and take note of which pages are generating 404 "Page Not Found" errors. Set up redirects for these URLs to point them to the new location.

With a little foresight and planning ahead, you can make the transition to your new site smoother for your users and retain as much search rank from your old site as possible.