Quoting Sean Goedecke

I’ve long thought consistency is king - I think this applies in codebases of all sizes, not just those in the single digit millions as Sean describes. Here’s the summary, though the full article is worth a read:

Large codebases are worth working in because they usually pay your salary

By far the most important thing is consistency

Never start a feature without first researching prior art in the codebase

If you don’t follow existing patterns, you better have a very good reason for it

Understand the production footprint of the codebase

Don’t expect to be able to test every case - instead, rely on monitoring

Remove code any chance you get, but be very careful about it

Make it as easy as possible for domain experts to catch your mistakes

Sean Goedecke