Setup A Default Featured Image

One simple method is to filter post_thumbnail_html, to add in a default image link:

<?php
function wpse55748_filter_post_thumbnail_html( $html ) {
    // If there is no post thumbnail,
    // Return a default image
    if ( '' == $html ) {
        return '<img src="' . get_template_directory_uri() . '/images/default-thumbnail.png" width="150px" height="100px" class="image-size-name" />';
    }
    // Else, return the post thumbnail
    return $html;
}
add_filter( 'post_thumbnail_html', 'wpse55748_filter_post_thumbnail_html' );
?>

You can make this filter more complex, but this should get you started.