You want to force the World Wide Web prefix of your domain name, even though a page was requested without it. For example, if you would like all requests to http://hans-eric.com be redirected to http://www.hans-eric.com

Solution:

Solution from Alister Cameron. It works for Linux/Unix and requires .htaccess file access.
In the web root directory .htaccess file, add the following:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www.alistercameron.com$ [NC]
RewriteRule ^(.*)$ http://www.alistercameron.com/$1 [R=301,L]RewriteCond %{REQUEST_URI} ^/[^.]+[^/]$

RewriteRule ^(.*)$ http://%{HTTP_HOST}/$1/ [R=301,L]
</IfModule>

If you are using WordPress, those rules should come before the standard WordPress rules.