Hypertext Access file Rewriting and Redirecting Adding a trailing slash at the end
Example
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$ /$1/ [L,R=301]
The first RewriteCond helps exclude the files. The second RewriteCond checks if there is already a trailing slash. If the case is so RewriteRule is not applied.
If you have any URL that shouldn't be rewritten, you can add one more RewriteCond.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteCond %{REQUEST_URI} !url/to/not/rewrite
RewriteRule ^(.*)$ /$1/ [L,R=301]
