Previous answers were good ones. To elaborate:
1. The simple way is to password protect a post. On that post put a link to your autoresponder subscription page, so that visitors who want the password will subscribe to your list.
2. The membership option is a good one. There are some good plugins (in fact I think there is a new one coming out about now) that transform your wordpress site into a membership site. Then you can allow members (subscribers to your list) access as you wish. Before purchasing a premium plugin check out the free ones at wordpress. (http://wordpress.org/plugins/membership/ I have not used this one, but it looks promising)   http://www.s2member.com/ I have used this one. It is very powerful, but there is a learning curve. If you go the membership route, take the time to understand the software that you use.)
3. The description of hiding posts in a directory inside the wp-content folder is very clever. Bo gets my vote. About hiding by ticking the hide button on the post/page settings: this option should be available for all posts, whether in a secret directory or not. There are some plugins that allow you to hide a page (I'm not sure about posts). What it means is, the page will not be listed on the nav-bar, or in the Pages widget. But it does not prevent access to someone who just happens to know the URL.
4. Have you considered hiding content, then opening it when someone subscribes to your list? A webpage may look like this: " .... a little content. Get access to this video when you sign on to my newsletter." Then you have a subscription form, the visitor subscribes, and the next thing they know, they are looking at your subscribers-only content. There are a few plugins out there that hide content until the visitor takes some kind of action. At the moment, I only know of premium plugins, but I would guess that there must be a free one that does essentially the same thing.