Ram used in interrupts

Use volatile:

volatile byte my_variable_name;

Stopping constants using RAM

Use the F macro to move string literals to program memory. Serial.print(“some string”) is stored in RAM (dynamic memory), Serial.print(F(“some string literal”) is stored in program memory.  Any function that uses the print class functions can use the F macro (e.g.client.print). 

Use progmem to move stuff to program memory.

Set array to value

byte data[100]
memset(data, 0, 100);