Once you want to keep data you need to think about memory use

Free memory

#include <MemoryFree.h>
void setup() {
    Serial.begin(115200);
    Serial.print("freeMemory()=");
    Serial.println(freeMemory());
}

Now you can see free memory, what can you do

Reduced buffers

Free up some of the serial buffer - http://www.ladyada.net/library/arduino/hacks.html

Don't use strings !

Arduino Static Strings