Ask HN: What are good startup habits, constructs or patterns to establish early?

Our startup is soon to expand from 4 people to 12 in 2019. We are an enterprise software company in the financial industry. The team will be roughly split between devs, quants and sales.

I am curious to hear from the HN community about habits, constructs and patterns that bring order to the chaos, keep people organized and motivated, manage requirements and keep people on track.

Are there particular pieces of software that have helped? I know everyone uses Slack - but there seem to be just as many detractors on HN. Our Slack usage has not reached a point where its distracting - but is it something I should watch out for? On the other hand, Slack is our monitoring tool - which, I know, isn't good.... But what are the alternatives?

Looking beyond software tools: How should I conduct meetings? Stand up? Sit down?! Etc...