See my post on http://onexa.nl/wordpress/aangepaste-url-voor-wp-admin/ for the use of this code (Dutch website)

.htaccess

RewriteEngine on
RewriteBase /

# WP automatically redirects domain.com/admin to the correct wp-admin URL, we want to stop this so we're sending the visitor to the homepage
RewriteRule ^admin/?(.*?) http://www.domain.com [R=301,L]

# Cloaking the wp-admin folder and changing it to domain.com/my-administration/
RewriteRule ^my-administration/(.*?) wp-admin/$1?%{QUERY_STRING} [L]

# Default WP stuff goes here -->

wp-admin-filter.php

<?php
/*
Plugin Name: Filter wp-admin
Description: Filter the site_url and get_site_url functions to cloak the wp-admin folder (+ remove the generator tag)
Version: 1.0
Author: Onexa - WordPress, webshops & meer
Author URI: http://www.onexa.nl
*/



/**
 * Function to filter the site_url and get_site_url functions to make sure the wp-admin links all reflect the changes we've made above.
 *
 * @var $url (string) - the URL that's getting filtered
 * @var $path (string)
 * @orig_scheme (string) - the type of link / what it's for (ie. login)
 **/
function onx_wpadmin_filter( $url, $path, $orig_scheme )
{
	$old  = array( '/(wp-admin)/' );
	$new  = array( 'my-administration' ); // the new "folder"-name for wp-admin, must be the same as given in the .htaccess & wp-config.php files
	return preg_replace( $old, $new, $url, 1);
} // end onx_wpadmin_filter
add_filter('site_url',  'onx_wpadmin_filter', 10, 3);


# remove the generator meta-tag to make sure you're not advertising you're using WP to ill-willing people / bots.
remove_action('wp_head', 'wp_generator');

?>

wp-config.php

<?php

# Put this just above the "Happy Blogging" line in your wp-config.php
define( 'ADMIN_COOKIE_PATH', SITECOOKIEPATH . 'my-administration' ); // <-- the "my-administration" part should be the same as in the .htaccess

?>