See http://arevico.com/templates-for-custom-post-types-and-posts/ on how to use this gist

custom-cpt-template.php

<?php
/**
 * Fast and Efficient Way to Include Template abilities for posts and custom post types
 *
 * Author H.F. Kluitenberg < Arevico.com >
 * How to use:
 * See: http://arevico.com/templates-for-custom-post-types-and-posts/
 */
class ArevicoTheme 
{

	/**
	 * Add all hooks and filters
	 */
	function __construct(){
		add_filter( 'template_include'	, array($this, 'metaTemplate'), 99 );
	}

	/**
	 * Switch the template when a meta tag (custom field) has been defined
	 *
	 * @param string $template The full path to the template file
	 * @return string the full path to the template file to be used
	 */
	private function metaTemplate( $template ){
		global $wp_query;

		// Only on pages and posts		
		if (! (is_page() || (is_singular() ) ))
			return $template;

		// Get a list of all registered templates in the form
		// { relative location } => { Template Name }

		$availableTemplates = wp_get_theme()->get_page_templates();
		$metaTemplate 		= get_post_meta( get_the_ID(), 'template', true);

		// If we didn't specify a template, just return the current template
		if (!$metaTemplate)
			return $template;

		// The template supplied is the php file
		if ( stripos($metaTemplate,'.php') !== false)
			return get_template_directory(). '/' . $metaTemplate;

		foreach ($availableTemplates as $path => $name) {
			if (strcasecmp($name, $metaTemplate) === 0 )
				return get_template_directory(). '/' . $path;
		}

		// else a template name has been supplied
		return $template;
	}


}