Talk with them regularly
A short one-on-one every week or two: how is it going, what is in the way, what do you need. Problems come up early instead of in a resignation letter.
Notice good work
Say thank you, and be specific about what was done well. Recognition costs nothing and is one of the reasons people stay.
Show a path forward
Good people want to grow. Talk about what the next step could look like, new skills, more responsibility, a bigger role, and what it takes to get there.
Keep pay fair
Check every year that pay is in line with the market for the role. Losing a good worker over a gap you could have closed is the most expensive mistake.
