Add to WordPress theme's functions.php or plugin. Changes the 'Enter title here' text for certain post types (most examples don't realise you can get the post type from the filter.

wp-change-title-placeholder.php

/**
 * Change the title placeholder text
 */
function my_plugin_change_default_title( $title, $post ){
	if ( 'posttype' == $post->post_type ) {
		$title = 'Enter new title here';
	}

	return $title;
}
add_filter( 'enter_title_here', 'my_plugin_change_default_title', 10, 2 );