To force SSL logins and the administration interface, edit wp-config.php and add these before the "/* That's all, stop editing! Happy blogging. */" line:

define('FORCE_SSL_LOGIN', true);
define('FORCE_SSL_ADMIN', true);

/* That's all, stop editing! Happy blogging. */

You must set up Apache to use SSL of course.

A lot more info can be found here:

• Administration Over SSL « WordPress Codex.