Have you done simple things such as using byte vs int for value that are <=255?
If you have large arrays, that can save a lot of space.
Or things like compressing data, using a byte to store 8 0s and 1s:
byte dataArray[] = {0,1,1,0,1,1,1,0, -> 8 bytes
vs
byte dataArray[] = {0b01101110, -> 1 byte