I haven't develop any web app using flask but the practices which I follow in order to make my workflow smoother and productive are:
A good text editor makes your workflow faster and smoother. I use Sublime-text and make use of emmet along with some packages in order to speed up my workflow.
2.) Work flow
3.) Modular approach
I use to have a modular approach while developing any web app by dividing different components and functionality into modules so to easily identify the bug when something goes wrong.
General ideas that are relatively easy to make but cover a lot of ground in terms of knowledge :