how to force a form to not be cached?

Here's the form. It checks to see if the user has a USERNAME stored in the session, meaning they are logged in to a 3rd party system I am integrated with. If so it loads all the user information and uses it as default_values in the form.

Thanks for taking the time to help by the way. This has me stumped. If forms are always cached as you say, then I don't understand how your forms could display different #default_values based on run-time info. I can even put a die() call at the top of my form function and nothing happens.

Also, just to be clear, I am on Drupal 6.16.

function ff_users_info_form(&$form_state) {

  $form = array();

  $form['#tree'] = TRUE;
  
  //turn off form caching
  $form_state['no_cache'] = TRUE;

    
  //if we have a current user coming to the form show their info in the form
  if($username = $_SESSION['USERNAME']) {
    $user = ff_users_get_user_info($username);
  }
 
  $form['account_details'] = array(
    '#type' => 'fieldset', 
    '#title' => t('Account Details'), 
  );  
  
  $form['account_details']['first_name'] = array(
    '#type' => 'textfield', 
    '#title' => t('First Name'), 
    '#size' => 36,
    '#default_value' => ($user['first_name']) ? $user['first_name'] : '',    
    '#required' => TRUE,
  );  
  
  $form['account_details']['last_name'] = array(
    '#type' => 'textfield', 
    '#title' => t('Last Name'), 
    '#size' => 36,
    '#default_value' => ($user['last_name']) ? $user['last_name'] : '',    
    '#required' => TRUE,
  );  
  
  $form['account_details']['email'] = array(
    '#type' => 'textfield', 
    '#title' => t('E-Mail Address'),
    '#size' => 36,
    '#default_value' => ($user['email']) ? $user['email'] : '',    
    '#required' => TRUE,
  );      
  
  $form['account_details']['address'] = array(
    '#type' => 'textfield', 
    '#title' => t('Address'), 
    '#size' => 36,
    '#default_value' => ($user['address1']) ? $user['address1'] : '',    
    '#required' => TRUE,
  ); 

  $form['account_details']['address2'] = array(
    '#type' => 'textfield', 
    '#title' => t('Address 2'),
    '#size' => 36,
    '#default_value' => ($user['address2']) ? $user['address2'] : '',
    '#description' => '(if needed)', 
    '#required' => FALSE,
  );  

  $form['account_details']['city'] = array(
    '#type' => 'textfield', 
    '#title' => t('City'),
    '#size' => 36,
    '#default_value' => ($user['city']) ? $user['city'] : '',    
    '#required' => TRUE,
  );    
  
  $form['account_details']['state'] = array(
    '#type' => 'textfield', 
    '#size' => 10,    
    '#title' => t('State or Province'),
    '#default_value' => ($user['state']) ? $user['state'] : '',    
    '#required' => TRUE,
  );      
  
  $form['account_details']['zip'] = array(
    '#type' => 'textfield', 
    '#size' => 10,    
    '#title' => t('Postal Zip Code'),
    '#default_value' => ($user['zip']) ? $user['zip'] : '',    
    '#required' => TRUE,
  );   

  $form['account_details']['country'] = array(
    '#type' => 'textfield', 
    '#title' => t('Country'),
    '#size' => 36,
    '#default_value' => ($user['country']) ? $user['country'] : 'USA',    
    '#required' => TRUE,
  ); 
  
  $form['account_details']['phone'] = array(
    '#type' => 'textfield', 
    '#title' => t('Telephone Number'),
    '#description' => '(area code first)',
    '#size' => 36,
    '#default_value' => ($user['phone']) ? $user['phone'] : '',    
    '#required' => FALSE,
  );   

  return $form;
}