I just did this myself because I was annoyed with having to explain to people how to view the site and it was taking too many steps. There might be a plugin or some easier way to do this, but I simply edited the wp-login.php (the login screen).
The wp-login.php is located in the base directory of your WordPress installation.
Look for the $redirect_to tags. There are several in there for different reasons, but the part of code I changed was.
$user_login = '';
$user_pass = '';
$using_cookie = false;
if ( !isset( $_REQUEST['redirect_to'] ) )
$redirect_to = '/wp-admin';
$redirect_to = $_REQUEST['redirect_to'];
$redirect_to = preg_replace('|[^a-z0-9-~+_.?#=&;,/:]|i', '', $redirect_to);
change the /wp-admin to just /