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 Bit Twiddling Hacks - 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.