Use consistent coding conventions, automatically enforced

Code should be formatted the same way and enforced automatically using tools.

Why

Code is read much more often than it is written. The more familiar the conventions the less time is spent on code reviews, visual parsing and code comprehension.
Code conventions are learned quickly. Automatic enforcement exposes the developer repeatedly to the same errors making learning comfortable and at their own pace.
Less mistakes. Code errors are raised straight away. In some cases they are fixed automatically.
Constrains choices. More focus on engineering problems rather than distracted by inconsistent code or uncommon language features.
Reduces conflict. Minor decisions, such as spacing and formatting, no longer need to be debated.

Use a static language to prevent errors.
Use a linter (e.g. eslint, python) or formatter (e.g. gofmt) to enforce standards.
Adhere to common language conventions or standards (e.g. Python's pep8).
In languages where a common convention hasn't been decided. Opt for popular third party configurations. There will be a higher chance that this will make third party libraries easier to read and the rules can be reduced to suit the teams preferences.
Link tools to IDE/Editor for rapid feedback.