Ask HN: Software development best practices?

I've been writing software professionally now for about a decade, but I feel like I've never learned how to do it 'properly'.

For example, I've never experienced agile methodology, never used unit testing on my code, and only just recently started using source control tools.

What are the current best practices I should consider following? And conversely, what worst practices and non-best practices should I avoid?

My projects have delivered just fine and my clients and employers have been happy with the work I've done, but I feel like I'm missing important techniques. I've mostly worked solo rather than on a team of developers so that might be a contributing factor.