General Dos and Don'ts

Dos

Do keep all code, regardless of its size, in a GitHub repository so the whole team can access it.
Do include a README and add code comments.
Do use Markdown, not Rdoc.
Do indent code with spaces, two spaces per indentation.
Do configure your text editor to use LF newlines. This is default on almost all text editors on macOS and Linux. On Windows, some text editors are set up to use CRLF (also called 'DOS format') newlines.
Do configure your editor to add a newline at the end of a file. Most editors enable this by default.
Do prefer one-off rake tasks for maintenance and migration over code in your database migrations.
Do loosely follow TomDoc for commenting. Combine this with comments within methods for particularly tricky bits of code.
When working in Ruby on Rails, do use Brakeman to perform static analysis for security vulnerabilities.
Do write recurring tasks as Sidekiq jobs and schedule them using sidekiq-cron. This has proven time and again to most reliably cover all the weird edge cases.
Do use squoosh to compress images and pick appropriate formats for them.
Do set config.time_zone in new Rails applications to the appropriate time zone for the client (e.g., 'Europe/London').
Do ensure there is centralized logging in place for every new project. As a team, we prefer Papertrail.
Do ensure there is exception handling set up for every new project. As a team, we prefer Rollbar, but we like to use Bugsnag for projects centered around a React Native mobile application.

Don't use the Heroku Scheduler; it doesn't guarantee a run, and we've had business-critical jobs fail.
Don't allow master/main builds to fail. If they do, ideally, everyone should stop what they're doing until it's passing again.