Short answer
Give them a real stretch, a visible path and time with senior leaders. High-potential people tend to leave when they stop growing or can’t see where they’re headed. A development program where your own executives teach them, and where they work on real cross-functional problems, does all three at once. Pay matters, but it’s rarely the only reason a strong performer walks.
The longer answer
The people you’d promote next are often the ones most likely to leave, because they have the most options. Many companies only find out who their high potentials were in the exit interview.
Name them. Tell them. Give them work that stretches them and access to the people who decide their future. And make sure their own manager isn’t holding them back to protect the team’s results.