See this useful writing guide by Matteo Carandini.
Practice version control; I like Google Docs, which works well with Zotero.
I’m a sucker for nice-looking figures (and fonts). Learn how to make your figures look good straight out of Python/Matlab/Julia/R, and minimize the time spent changing things in Illustrator/InkScape. This will not only save a lot of time in the long run, it also makes your analyses more reproducible.
Scientific autorship is a tricky business (see here for discussion in my PhD lab). I aim to discuss authorship early on, clarify expectations, and give regular updates when a project changes. If you are unsure about authorship, please let me know asap.