I used the following to require the checkout section of a website to require SSL:
<Directory "/var/www/html">
RewriteEngine on
Options +FollowSymLinks
Order allow,deny
Allow from all
RewriteCond %{SERVER_PORT} !^443$
RewriteRule \.(gif|jpg|jpeg|jpe|png|css|js)$ - [S=1]
RewriteRule ^checkout(.*)$ https://%{SERVER_NAME}%{REQUEST_URI} [L,R]
</Directory>
So for example, hitting http://www.example.com/checkout redirects to https://www.example.com/checkout
The rule will skip file extensions that are typically included within a page so that you don't get mixed content warnings. You should add to this list as necessary.
If you want multiple pages change the RewriteRule to something like:
RewriteRule ^(checkout|login)(.*)$ https://%{SERVER_NAME}%{REQUEST_URI} [L,R]
Of course, the directory should match the actual path on your server. This page may also help with some more information for your specific needs: http://www.whoopis.com/howtos/apache-rewrite.html
I'm using this on a website that runs Plesk 8.6 but that shouldn't matter. This is in my vhost.conf file which is like putting it in your httpd.conf file. I'm not sure if you'd need to adjust anything to use it in a .htaccess file but I doubt it. If adding to a conf file don't forget to restart apache to reload the configuration.
If you are like me and want to use SSL only on particular pages then you also want a rewrite rule that sends you back to regular http for the rest. You can use the following for the reverse effect:
RewriteCond %{SERVER_PORT} ^443$
RewriteRule \.(gif|jpg|jpeg|jpe|png|css|js)$ - [S=1]
RewriteRule !^(checkout|login)(.*)$ http://%{SERVER_NAME}%{REQUEST_URI} [L,R]
If you are using Plesk like I am keep in mind that all non-SSL traffic uses the vhost.conf file but all SSL traffic uses the vhost_ssl.conf file. That means your first rewrite rule to require SSL would go in the vhost.conf file but the second rule to force back to non-SSL will have to go in the vhost_ssl file. If you are using httpd.conf or .htaccess I think you can put them both in the same place.
I've also posted this tutorial on my blog: Apache rewrite rules to force secure/non-secure pages.