#### No https except to wp-admin -
# If the request is empty ( implies fopen or normal file access by a php script )
RewriteCond %{THE_REQUEST} ^$ [OR]

# OR if the request if for wp-admin or wp-login.php
RewriteCond %{REQUEST_URI} ^/(wp-admin|wp-login.php).*$ [NC,OR]

# OR if the Referer is https
RewriteCond %{HTTP_REFERER} ^https://www.askapache.com/.*$ [NC]

# THEN skip the following rule, basically all this does is force https or badhost to be redirected
# BUT because of the above 3 rewritecond's, this won't break poorly written admin scripts
RewriteRule .* - [S=1]

RewriteCond %{HTTPS} =on [OR]
RewriteCond %{HTTP_HOST} !^www.askapache.com$ [NC]
RewriteRule .* https://www.askapache.com%{REQUEST_URI} [R=301,L]

RewriteCond %{THE_REQUEST} ^[A-Z]{3,9} /(wp-admin/.*|wp-login.php.*) HTTP/ [NC]
RewriteCond %{HTTPS} !=on
RewriteRule .* https://%{SERVER_NAME}%{REQUEST_URI} [R=301,L]

Comprehensive .htaccess example file with advanced examples in 1 htaccess sample skeleton .htaccess file with the very best apache htaccess examples... Updated frequently based on detailed info from the Apache htaccess tutorial.