Htaccess https force

Update:
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

When using ssl offloading:
RewriteEngine On
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

RewriteEngine on
RewriteCondition %{SERVER_PORT} !^443$
RewriteRule ^(.*)$ https://www.example.com/$1 [R=301,L]

RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://www.example.com/$1 [R=301,L]

source: http://serverfault.com/questions/116206/how-do-i-use-htaccess-to-always-redirect-from-http-to-https