If WordPress asks you your FTP host, username and password each time you’re trying to upgrade, you should eaither check your file permissions or use this very useful hack which will make WordPress remember your FTP host, username and password.

define('FTP_HOST', 'ftp.yoursite.com');
define('FTP_USER', 'Your_FTP_Username');
define('FTP_PASS', 'Your_FTP_password');
define('FTP_SSL', true); // If you can use a SSL connection set this to true