For the love of God, critically think about your database schema and object model at a very detailed level, taking into account likely future requirements. I can’t emphasize enough how important having the correct data shape is from the beginning. This is something you cannot ‘iterate on.’ You have to nail it from the get go, especially the parts central to your application. It’s either right or it isn’t and as soon as it is baked throughout your entire codebase it is too late to change.

I am currently refactoring a giant system that had a badly normalized object model that couldn’t be extended and have been doing so for over a year.