Enable SSL for logins and admin pages in wordpress:
http://codex.wordpress.org/Administration_Over_SSL
enable SSL on your Apache server:
http://osinside.blogspot.com/2007/08/how-to-centos-apache-server-with-ssl.html
http://www.debian-administration.org/articles/349
install WordPress HTTPS http://mvied.com/projects/wordpress-https/
- check Internal HTTPS Elements
- check Disable Automatic HTTPS
further steps (not tested by me): Enable complete support for SSL on WordPress