Sure strings can be used from Flash. And maybe other things can be PROGMEMed.
But will this help in this particular case, as the bottleneck is the 32K memory of the mega328 ?
Another direction is to question the real need for all libraries : the included libraries probably eat up already a large part of(program) memory : SD, Wire, SoftwareSerial, SerialLCD, floating math for temperatures, etc.