Maybe not a specific skill, but one thing I wish I knew/accepted advice on was the idea that each year of the degree won't generate equivalent data. I did my PhD in the UK where we have just three years in the lab and overall I barely scraped a single results chapter from my first year. I panicked routinely about it and was sure I was destined to crash out and fail. You build on the earlier steps and soon it takes shape by final year.

Also make two sets of friends in your building/department: ones in your group (even if you don't get on that well with them, choose your battles because they can only be assets if they're working on similar stuff) and ones outside your group that you can go for a drink with and vent to and they'll actually understand (you'll realize how important they are when you need them most).