String to persistent memory (EEPROM?)
EEPROM library has two functions put and get , which can help you store character array of any size, not exceeding the size of internal EEPROM size of Arduino. Please see the examples provided in the link above.

Here is a sample code to get you going.

int address = 10;
char arrayToStore[20];                    // Must be greater than the length of string.
String testString = "hello there";

testString.toCharArray(arrayToStore, testString.length()+1);  // Convert string to array.

EEPROM.put(address, arrayToStore);                 // To store data
EEPROM.get(address, arrayToStore);                 // To read data