What was the most useful thing you learned during your phd - and what where the biggest mistakes? What did you learn the hard way?

Gaining confidence was the hardest (and most useful) part for me. For the first 4 months I was convinced I was really stupid, had somehow fluked my way in, and was bound to be found out at some point or another. It took a while to work out that I must have got here on my own merits and that my supervisors weren't super-intelligent beings who I couldnt ever hope to talk to on the same intellectual level - they were just people who had worked hard and used their brain!

I'm starting my PhD in October. I'm feeling exactly the same, that I fluked my way in. Thanks for the advice

The most useful things I learnt: realizing that there can be many different ways to address any given question/problem. Also, if you can get through a phd, (esp. if you have an absolutely useless supervisor like i did) you can get through just about anything. What did i learn the hard way? Sometimes the most important things you learn from doing a PhD/research are not scientific.

patience
proaction
power of silent smiling
magic word: NO

the hardest thing on my side is being able to be stable in my motivation rather than 5% to 200%