If you are porting over an existing website to Drupal, one consideration is how you redirect the old page URLs to the appropriate pages on the Drupal version of the site. If you don't want to create custom rewrite paths within Drupal for those nodes -- or perhaps cannot due to clean URLs or filename suffixes -- then 301 redirects are considered the best way to handle redirected pages, for they inform search engines to update their databases with the new paths. This way, you should not risk your search engine pagerank or lose site visitors with 404 "not found" errors.
However, 301 redirects cannot be done using the common approach. Yet establishing 301 redirects is quite easy, provided you have mod_rewrite enabled in your .htaccess file.
How to create 301 redirects in Drupal Apache mod_rewrite
Edit your .htaccess file in a text editor. [Note: Be sure to save the file in "UTF-8" format.]
In the file, you will find the commands:
# Various rewrite rules.
# Modify the RewriteBase if you are using Drupal in a subdirectory and
# the rewrite rules are not working properly.
Immediately after that code -- and before the Drupal-provided "Rewrite old-style URLs" commands -- add your rewrite rules using the following format:
RewriteRule ^old/URL/path$ http://example.com/new/path [R=301,L]
#end custom redirects
Note the convention: The old path is simply the path off the root. The new path is the full path, including the domain. The [R=301,L] code is the 301 redirect instruction. (axbom notes: "The 301 tells browsers and spiders it is a permanent redirect, and the L ensures that no other rewrites are processed on the URL before it reaches Drupal; Hence place this code above Drupal 's own URL rewrite, but below the command RewriteEngine on.")
If you have more paths to add, insert the rewrite commands as their own line as above.
For more information, see the forum thread at http://drupal.org/node/16084 [from where I drew this information]