define(\'FS_METHOD\', \'ftpext\');
define(\'FTP_BASE\', \'/path/to/wordpress/\');
define(\'FTP_CONTENT_DIR\', \'/path/to/wordpress/wp-content/\');
define(\'FTP_PLUGIN_DIR \', \'/path/to/wordpress/wp-content/plugins/\');
define(\'FTP_PUBKEY\', \'/home/username/.ssh/id_rsa.pub\');
define(\'FTP_PRIKEY\', \'/home/username/.ssh/id_rsa\');
define(\'FTP_USER\', \'username\');
define(\'FTP_PASS\', \'password\');
define(\'FTP_HOST\', \'ftp.example.org\');
define(\'FTP_SSL\', false);

http://digwp.com/2010/11/ftp-in-wpconfig/