Most of what made me a decent PM, I picked up before I was a PM.
Years of web development taught me plenty about browsers, deadlines and how many ways a client can say "can you make the logo bigger". But the habit that's paid off most has nothing to do with code.
Get on well with as many people as you can. Especially the ones you don't think you'll ever need.
You never know when
As a developer, you're constantly relying on people outside your bubble without realising it. The person who knows where the old hosting details live. The account manager who can soften a client before you deliver bad news. Someone in another team who built something similar two years ago and could save you a week.
You can't predict which one you'll need, or when. That's the whole point. If you only build relationships with people who are useful to you right now, you'll find yourself stuck at the worst possible moment, asking a favour from someone you've barely said hello to.
And the reverse is worse. Burn a bridge because you've decided someone's irrelevant, and it has a funny habit of coming back to bite you. People move teams. They get promoted. They end up sitting between you and the thing you need, and they remember exactly how you treated them when you thought they didn't matter.
The PM version
This matters even more once you're managing projects. A PM's real output is getting things to move, and things only move through people.
A project plan tells you who's supposed to do what. Relationships decide whether they'll pick up the phone when the plan falls over on a Thursday afternoon.
The slightly unpopular bit
PMs who proudly say "I don't do office politics" aren't above it. They're just bad at it.
Politics, in the useful sense, is just knowing people, understanding what they care about, and having enough goodwill banked that you can ask for help without it feeling like a transaction. If you avoid that entirely, you haven't stayed clean. You've just made yourself less effective and left the field to people who are happy to play.
The transactional version is just as bad. Everyone knows the person who only ever messages when they want something. Nobody rushes to help them.
It scales up, too. Businesses that treat certain teams as overhead, and only notice them when something breaks, shouldn't be surprised when nobody's falling over themselves to fix it.
It's not a networking strategy
None of this needs a system. It's chatting to people while the kettle boils. Remembering what someone mentioned last week. Helping out when it costs you ten minutes and gains you nothing obvious. Saying thanks properly, and in public where it counts.
If it feels like a strategy, people will smell it. It only works if you actually mean it, which, handily, also makes work a lot more pleasant.
Good PMs know who to ask. Great PMs already have someone happy to answer.