There is an extension for deleting test orders
http://www.magentocommerce.com/extension/reviews/module/873
or see this
http://www.eliasinteractive.com/blog/magento-ecommerce-how-to-reset-all-test-order-information-and-set-unique-prefix-for-orders-invoices-shipments-and-credit-memos
or this one http://inchoo.net/ecommerce/magento/delete-test-orders-in-magento/