Paste this into your .htaccess file
# BEGIN Force SSL
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTPS} =off
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
</IfModule>
# END Force SSL
I once had the needed to exclude a directory, so http:// versions would still be served. This was just one extra line:
RewriteCond %{HTTPS} =off
RewriteCond %{REQUEST_URI} !^/directory/
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]