Я нашёл три варианта:
RewriteEngine On RewriteCond %{HTTPS} off RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
RewriteEngine On RewriteCond %{SERVER_PORT} !^443$ RewriteRule ^/(.*) https://%{SERVER_NAME}/$1 [L,R]
SSLRequireSSL Redirect permanent /secure https://www.domain.com/secure