return to the previous page after login. Template override com_users/login/default_login.php for Joomla!

default_login.php

<?php
if ($this->params->get('login_redirect_url'))
{
	$login_redirect_url = $this->params->get('login_redirect_url', $this->form->getValue('return'));
}
else
{
	$referer            = htmlspecialchars($_SERVER['HTTP_REFERER']);
	$uri                = JUri::getInstance($referer);
	$query              = JFactory::getApplication()->getRouter()->parse($uri);
	$login_redirect_url = 'index.php?' . JUri::getInstance()->buildQuery($query);
}
?>
<input type="hidden" name="return"
       value="<?php echo base64_encode($login_redirect_url); ?>"/>

feel free to improve when visitor access the page directly to login screen. ;-)