Very true, smal is beautiful
Some rules of thumb:
use small texts for your display, minimize debug messages length, use uint8_t if you only need a value between 0..255 also in a for loop.
Also learn to use bit manipulations like & | ^ ~
see tutorial section for a short intro, and http://graphics.stanford.edu/~seander/bithacks.html
- for advanced bithacks.
And don't use Strings but char array as Strings can be very RAM expensive
use functions with local vars iso global vars. Functions keep your code clean and leads to reuse of code.