<?php
$order = Mage::getModel('sales/order')
->load(Mage::getSingleton('sales/order')->getLastOrderId());
$getLastOrderIncrementId = $order->getIncrementId();
If the above code does not work try the following below:
<?php
$order = Mage::getModel('sales/order')
->load(Mage::getSingleton('checkout/session')
->getLastRealOrderId());
$getLastOrderIncrementId = $order->getIncrementId();