If you're working a lot of stuff, you'll come across it sooner or later -- "learn to say no."
It's true. You need to learn to say no. There's a limited number of labor hours you've got, and a limited number of calendar days you've got, and a limited amount of mental bandwidth, and a limited amount of scarce resources. Every hour, day, thought, and dollar you deploy in one area is not deployed in another area. Not only should you not attempt to do everything, you should recognize that trying would be crazy.
I know. I empathize. When you're chaining a run of successes together and everything is going smoothly, it's oh-so-tempting to layer more and more on.
So, learn to say no?
Yes, you should learn to say no. But it's more than that.
I'd go as far as to say it's a skill. A skill of 'Passing.'
Passing would be the ability to gracefully say no, to set boundaries and not take on commitments, and to elegantly disengage from bad engagements, not throwing good money after bad money and good time after bad time.
But more than that, it's a skill related to having a base of experience of knowing when you should pass. There's some fast-completion campaigns that are most likely duds, but have a small chance of being huge gems. If they're really fast-completion (and not masquerading as fast-completion with hidden pitfalls), it can make sense to bet on one of those duds. A number of the stupider, crazier, whackier-seemingly ideas you dare to take a flyer will pan out to be huge wins. Being right alone isn't enough to win big, it's usually necessary to be right against the grain and capitalize on that position.
That means 'passing' is harder than it looks. The natural inclination is pass on eccentric-looking stuff, which is perhaps entirely right for preserving cash and capital, but entirely wrong to not deploy a bit of time here and there and seem if you can make a counter-cultural breakthrough.
Passing is probably most important to deploy on things that a half-assed commitment would produce minimal results. There's a number of pursuits that are incredibly worthy when studied carefully, like picking your own stocks. You can do very well with it if you study it careful and fully understand the difference between speculating and investing (principle protection and safety), understand fully a margin of safety, understand dividend yields, learn to understand why some sectors become sexy and some sectors become unsexy (and buy the latter), and especially crucially -- learn investor psychology so that you can keep your head when there's blood in the streets... and even buy more...
Well, it would be good. But if you don't have the discipline to start with Ben Graham's The Intelligent Investor (Warren Buffet called it the best book on finance ever written), and then carefully apply Graham's principles to financial statement after financial statement after financial statement...
...then you'd be better off to stay out of that game.
Learning how to buy stocks correctly, and all the relevant principles, takes a lot of time. If you'd want to do it casually, don't bother.
And many things fit in that category. There's a number of fields and disciplines where you can make tremendous gains with a highly focused effort of between 7 and 60 highly dedicated, high-quality hours per week. But, if you engage in those fields sporadically, you get not a dang thing done.
There's 10,000 other considerations. Some one-off projects have people that are cooperative, encouraging, inventive, and responsible. Some projects might pay more for seemingly less time, but become a huge mental drain.
Some secondary projects, hobbies, and initiatives harmonize incredibly well with what you're currently doing, and lead to insights and innovations in your core pursuit. Others suck up your mental time while giving no great payoffs in material gain. And some of those aren't even very rewarding emotionally!
So, sure, learn to say no. But that's probably the easy part. The hard part is knowing what to pass on.