Add trailing slash to URL

[join]

I'm a newbie in web-development
How can I modify .htaccess file so that to add trailing slash to every URL?
http://www.site.com/about/ instead of http://www.site.com/about

My .htaccess file looks like:

Code:

AddDefaultCharset UTF-8
RewriteEngine on

RewriteBase /getsimple/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule /?([A-Za-z0-9-]+)/?$ index.php?id=$1 [L]

Thanks in advance.