I wrote a series of articles on the topic. You can check it here.
Here are the few points that seem important to me in the end:
Do systematic code reviews (but it's not enough)
Give complex tasks also to the juniors but help them
Stay close to the domain of your client
Use high level tests reflecting your domain
Write explicit code
Detect a lack of shared knowledge as soon as possible, and write down everything in your repository/code. Stop this oral culture: act as if your private project was an open source project on which every unknown developer should be able to collaborate
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I wrote a series of articles on the topic. You can check it here.
Here are the few points that seem important to me in the end: