I've seen this question asked several times in the past month and the long and short of it is, unless you use a service or manually back them up or save them to your SIM card there is no way to retrieve deleted messages. There are programs for PCs and Laptops that say they can recover deleted messages, the way you would deleted files. The caveat to that is the space they occupied is the same space that your new texts now occupy and therefore the longer you go without attempting a retrieval while receiving new messages the less likely much of anything will be recoverable.