The importance and power of saying “no.” Like most of us, I’m faced with a constant stream of opportunities and things to do, and I tend to say “yes” more often than I should. But because I haven’t managed to bend the laws of space and time, there’s a limit to what I can reasonably do—let alone do well. So before I agree to add something new, I ask myself what I’m going to subtract. If I can’t make room, I say no.

I also wrote a simple blog post several years ago that changed my life. It explains my professional priorities—HubSpot is the first one by a long shot, but the time I have left goes to activities that I hope will help the most people, like blogging, public speaking and angel investing. The blog post also offers some resources for the questions I get most often. Now when someone reaches out to me with a business opportunity or startup idea, I share the post with them, so they know that I would love to say “yes,” and also why I can’t.

Find an exceptional co-founder. Startups can be a lonely journey, and you need someone who shares your values, but also has a complementary background and complementary skills. My HubSpot co-founder, Brian Halligan, and I are different in many ways. He grew up in the U.S., while I spent my formative years in India. He’s a big Boston sports fan; I’ve never been to a game. He has a sales background, I have a product/engineering background. But we both believe in transparency and the importance of company culture, and we both obsess over customers rather than competitors.

Having a co-founder also dramatically mitigates the risk that you’ll go too deep down a rabbit hole, because there’s at least one other person you have to convince. I was once smitten with a project called “YouSpot,” which was HubSpot for solo entrepreneurs. The idea might have been good, but Brian rightfully convinced me that the timing wasn’t—HubSpot was already doing a lot, and expanding into that new use case would have been too much. He’s a pragmatist and a good counterweight to my sometimes overly ambitious ideas.

Years ago, I decided I was going to avoid phone calls if at all possible. I have an almost pathological aversion to them, for several reasons—I hate making small talk, I can be more thoughtful in an email, and I work much better in asynchronous mode. Now, I have fewer than 10 business calls in an average year. At first, I worried people would think I’m strange. And I am! Most people don’t have to emotionally prepare for a simple phone call—I do. But I made the change anyway, the world kept spinning and everything still gets done. My life turned out fine. It’s taught me that while there are always trade-offs in life, there are very few true “requirements."

How to play a musical instrument really well. I’m what you might call “music illiterate.” I can play some melodies by ear, but I can’t read or write music.

When I turned 50, I decided to try to fix that, so I’m teaching myself to play the piano. Right now I’m taking online courses from Simply Piano, Playground Sessions and Yousician, each of which brings something different to the table. It’s going pretty well so far, though I do get stuck for longer than I’d like on certain skills. It took me days to get through the chapter on “broken chords,” and I still haven’t quite mastered them. My ultimate goal is to learn at least 100 songs—everything from pop to classical to Broadway to kids’ music. And by “learn,” I mean be able to play them in a way that is by some definition recognizable.

I switched to e-books years ago, so they’re all on my Kindle rather than my physical nightstand. But the current list includes Michael Pollan’s How to Change Your Mind, which is exceptionally well-written; Gary Chapman’s The 5 Love Languages; and Platform Revolution, by Geoffrey Parker, Marshall Van Alstyne and Sangeet Paul Choudary, which is about how economics are shifting from traditional “pipeline” businesses to platform businesses like Airbnb and Uber.

I’m also reading Micro:Bit: A Quick Start Guide for Teachers, by Ray Chambers, because I’m trying to help my seven-year-old learn to code. I think it’s been harder for me than it is for him, because it’s a challenge to adopt a true “beginner’s mind.” I have to remember that he doesn’t know how to touch-type or use Excel, and he hasn’t learned math fundamentals like functions and Boolean algebra. So we’re sticking to visual programming languages for now, and he’s enjoying it. He likes being able to take ideas and turn them into “code.” Programming is a form of expression, just like any other creative endeavor.

Many times. One that jumps to mind, because it was among the most painful, was when I started my second company while still being CEO of my first. I thought that since I had a management team in place at the first company, it would work out, but I was wrong. Startups are just so all-consuming. There are maybe a handful of people in the world who can run two companies at once, and I’m not one of them.

I ended up merging the two companies, selling that company and going back to grad school. That experience taught me that it’s okay to sell something if your heart’s just not in it. There are always more ideas. Plus, I think liquidity events are highly underrated. Money can’t buy happiness, but it can buy security and create options. Selling that company opened up so many other paths, including the opportunity to write the seed check for HubSpot.