How can one retrieve deleted text messages?

If the SMS messages are saved on the SIM card it's very easy, use a tool like CHIPDRIVER or , to view the haw SMS data, change the header to mark it as current instead of deleted. http://3d2f.com/tags/undelete/sms/messages/

From a mobile phones internal memory, this is a little tricky, there are some techniques you can use, such as service boxes and/or Flasher boxes (e.g. SHU Box). These will do direct memory reads from the phone, but you will need to work out how to interpret the data.

Mobile phones use EEPROM/NAND memory; this is Electrically Erasable Programmable Read-Only Memory.

Wear Leveling Algorithms are used to extend the life of the memory, this causes issue when reading data directly from the memory chip, as it needs to be re constructed. In many cases it is FAT file system.