How do I add some static routes to a CodeIgniter app?

Modify your .htaccess like so:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php/$1 [L]

RewriteCond %{REQUEST_FILENAME} !-f tells mod_rewrite ignore the url if it's an actual file, and RewriteCond %{REQUEST_FILENAME} !-d tells it to ignore it if it's a directory.