(This post is part of a bigger list of rules that I have found helpful for thinking about a career, and beyond. See this post for an explainer).
Ask anyone in more advanced stages of their career about their biggest weaknesses in their professional lives, and you'll very often hear the phrase "I say yes too often".
So here is a simple rule: say no more often.
It sounds like bad advice. Shouldn't we be more open? Shouldn't we welcome new opportunities? Shouldn't we be excited if we are asked for input? Yes! Yes, we should be, but the more choices we have, the more selective we have to be.
Your time is very limited. Your time of full concentration is even more limited. The real problem is that for every yes, you're taking away your resources from other things that you also said yes to. If you say yes to too many things, you either won't be able to give your projects the attention they need, or you'll disappoint people you said yes to previously (or both). Either way, it's bad.
"But isn't my CV more impressive if it has lots of stuff on it? The more, the better?" you may ask, especially early in your career. The advice I'd give here is the same as the advice I'd give on how to prepare a presentation - most people will be able to take away one thing from it, a few may be able to walk away with 2-3 things. That's it. I think the same is true for a CV - after some basic vetting, you will be mainly associated with one thing that you did exceptionally well. The thing that truly stood out. The thing nobody else did.
This reminds me of one of the many great pieces of startup advice that YC gives: "We often say that a small group of customers who love you is better than a large group who kind of like you." I would argue the same is true when people decide whether to hire you or fund you. If everybody feels OK about your work, you're in trouble. If you have a few people who love one or two things you did extremely well, you'll be doing much better - they'll be your champions. I know this flies in the face of the advice some people give, especially in academia, which boils down to "just be sure to have all checkboxes ticked off, and don't show any weaknesses". All I can say is these people are wrong. Of course, if you're looking to spend your working life at incredibly boring places, you should follow these rules. Which, coincidentally, reminds me of yet another great piece of advice I heard around YC: When looking for brilliant people, look for the presence of strength, not the absence of weakness.
What does this have to do with saying no? Simple: you can't do something great unless you devote very large chunks of time on it. With every yes, you dilute yourself. So be careful when you say yes. Say yes only to things you can absolutely commit to, and no to everything else. Don't feel bad about saying no - you're really saying yes again to the things that you've already committed to.