Let me think:

• do not repeat yourself aka create functions
• use markdown! this is probably the most important one as it is one of the big advantages of using a notebook
• try to use libraries (or .py files) to group code which is used often
• use repos
• and this is personal taste and perhaps controversial but use python or scala (not SQL)
• and a lot of stuff I probably forget, but other people will add this 🙂