Force trailing slash for SEO purposes using htaccess 301 redirects (mod_rewrite) - but on GET requests only, to avoid losing POST data on a POST request to a URI without a trailing slash.

.htaccess

# Force trailing slash for SEO purposes
RewriteEngine On 

# For GET and HEAD requests only. We do not want to redirect posted forms and such, or we'll lose all POST data!
RewriteCond %{REQUEST_METHOD} ^(GET|HEAD)$

# Not for actual files. We do not want to redirect urls like test.jpg to test.jpg/
RewriteCond %{REQUEST_FILENAME} !-f  

# Redirect to trailing slash if no slash is present in URI
RewriteRule ^(.*[^/])$ /$1/ [L,R=301]