When I started Barnes & Conti in the early 1980s, I had a head full of theory and a calendar full of people who were kind enough to let me try things on them. Some of it worked. Much of it didn't. The list below is the short version of what stuck.
On leadership
The job is not to have the answer. The job is to make it safe for the people who do to say so out loud. The job is not to tell people how to get somewhere. It is to show people where you would like them to go and provide them with the support they need as they find their way.
On influence
You don't get more of it by being louder. You get more of it by being clear about the destination and flexible about the means to achieve it. [Most arguments fail not because the position is wrong but because the framing is sloppy enough that nobody trusts it.]
On consulting
If the client can't describe the problem in their own words, you don't yet have a problem. You have a symptom. [Diagnose the symptom long enough and the problem usually surfaces on its own.] Deep listening and questioning can uncover the underlying issues so you and your client can work on solving them.
On risk
Risk that's been thought through is rarely the thing that takes you down. The thing that takes you down is the risk you didn't bother to think through because you were too busy being decisive about something else. Intelligent risk-taking means considering gains, losses, implications and consequences - and then actively managing them.
On innovation
Most organizations don't have an innovation problem. They have a debate problem. New ideas don't survive the trip from one person's head to the room where they're supposed to land, because the room isn't set up to receive them. Fix the room. The ideas will follow.
On your career
The work you do at 30 is usually not the work you should be doing at 50. If it is, you are either very lucky or very stuck. Either way, it's worth a conversation with yourself about whether it's still the right work.
I've written longer versions of all of these. But if I had to send a younger me a single page, it would be this one.