Reusability of code
After having copied code snippets of your older projects again and again it's time to find out how to write a lib of your own.
You will find instructions on the net on how to do it.
Still, in the beginning it is hard to start with it.
Have a close look at the libraries that are shipped with the Arduino system.
Take track of your projects that make use of your self-written libraries.
Once you update one of your libraries better check if the dependent projects are still working.
To support multiple usage of your libraries make use of conditional compilation.