If there are certain posts that you want to restrict, whether they for a few people only, or for paying subscribers, or whatever, you can use this snippet to force users to login to see them. Paste this into your functions file:

function my_force_login() { 
global $post;
if (!is_single()) return; 
$ids = array(188, 185, 171); // array of post IDs that force login to read 
if (in_array((int)$post->ID, $ids) && !is_user_logged_in()) { 

And then put this at the top of your header:

<?php my_force_login(); ?>