Following on from Phil's comments - it automatically does it. You receive your standard SMS tone and vibration, and if you don't acknowledge the receipt of the text (by closing it, viewing it, or replying to it) then the phone will alert you to it again after 5 minutes, and then after another 5 minutes. The second and third notifications are only audible and the phone vibrates (if you have your phone set to do this). The screen will stay black on further notifications.