I recently followed a course on "Principles of programming for Econometrics" and although I knew a lot about programming already I learned a lot about being structured and documentation. The professor ran some example code which he wrote 10 years ago! He wasn't really sure what the function did again and BAM it was there in the documentation (i.e. comment header of the function).

I used to just hack stuff together in either R or Python but that course really got me thinking about what I want to accomplish first. Write that down on paper. And then and only then after you have the whole program outlined in your head start writing functions with well defined inputs and outputs.