How do I retain key people through an acquisition or earn-out?

If my best people walk after the deal, my earn-out and reputation are sunk

Tie them in before the deal with equity or a stake in the upside, give them a reason to stay beyond money (role, growth, ownership of the future), and be honest with them about what the deal means. Buyers are partly buying your team's commitment, so retention you've secured in advance protects both the price and your earn-out.