Changing custom variables between pages?

I want to setup a custom variable to highlight people who sign up multiple times at the site. So, when the user signs up for the first time, I record a custom variable for that:

$customer_action_tracker->setCustomVariable(1, 'Signup', 'New', 'visit');

Then, if he tries to sign up again during the same session, I want to read that variable and if it is set already - change it to:

$customer_action_tracker->setCustomVariable(1, 'Signup', 'Multiple', 'visit');

That doesn't work though - when the user loads a new page I create a new tracking object and it is blank:

print_r($customer_action_tracker->getCustomVariable(1, 'visit')); - prints an empty space, and not an array with Signup=>New as I expected.
What am I missing?