use a shortcode so you can set it for only the posts you want
add this to your themes functions.php file
Code:
add_shortcode( 'member', 'member_check_shortcode' );
function member_check_shortcode( $atts, $content = null ) {
if ( is_user_logged_in() && !is_null( $content ) && !is_feed() )
return $content;
return '';
}
then in your post
Code:
[member]
This text will be only displayed to registered users.
[/member]