Tips and tricks for Arduino

• Try to use less loops, less if statements and more calculations with less abstraction,
• think like procesor, less like a human (some things are better to be complicated but efficient),
• inspire by online guides and tutorials, don't copy them,
• libraries can lie to you, try to see their source code or header file for documentation,
• read official AVR datasheet for given microcontroller (helps a lot),
• don't worry about baremetal code, like read/write into specific PORT (it's fast and efficient),
• Serial is good for debugging, less for final product, try to evade him if you can and keep everything inside of microcontroller.