Notebook Best Practices

Use Collapsible Headings and Table of Content
Notebooks should be executable from top to bottom
Name your variables carefully
Use dummy names such as tmp or _ when needed
Clear useless variables when not needed (del my_variable)
Clear your code and merge cells when relevant (Shift-M)
Hide your cell outputs to gain space (double-click on the red Out[]: section to the left of your cell).