If you want to always add a slash at the end of your URL addresses, add this rule to your .htaccess file:
RewriteEngine OnRewriteBase /RewriteCond %{REQUEST_FILENAME} !-fRewriteRule ^.*[^/]$ /$0/ [L,R=301]
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^.*[^/]$ /$0/ [L,R=301]