Genuinely give a shit. That's the TL;DR but I'll elaborate. High turnover, especially at startups, is often the result of burn out. Here are my thoughts on preventing burnout.
Here's the thing, your employees care about your business. Even the lazy ones. But they don't want to fight fires all the time. Set the expectation that they should be intense while they're at work and have a life afterward[1]. Instead, set reasonable goals. Figure out what's most important and give them reasons to want to work hard for you. What incentives? Money. Food. Feedback[2]. Sure, any of those, and probably lots of other things.
Employees don't want to feel like "resources." They want to feel human and do their part to make the cogs turn. Training employees will cost you in the long run so give current employees good reasons to stick around. Don't be stingy. Give employees enough money to make them feel valued. That means good raises, bonuses, and so on [3]. While money isn't everything, the perception of forward movement goes a long way.
Food is great because it's a great way to make people feel cared for without spending tons of money, especially during crunch time. At my day job, they buy us lunch when it rains or when new people join. Sometimes it's fancy food and other times it's pizza. But it's always appreciated.
Feedback is free. Let employees know what you think of their work. If they're doing a great job then praise their work. If they're struggling then coach[4] them and help them improve. That's often easier than finding new employees to replace the ones you fire.
It's not your job to make your employees happy but it is your job to provide a great work environment, set expectations, and encourage employees to be intense at work without burning out.
[1] Sometimes work outside of typical hours is required. That's fine. What's not fine is for that to be the norm.
[2] You should also give critical feedback when your employees screw up. Employees need mentorship as much as they need praise.
[3] Giving extra money isn't always possible. Profitable companies (or companies with tons of VC dough) can do a better job at this. And obviously it should be a sound business decision (as defined by the resources you have at your disposal).
[4] Note that coaching and negativity are not the same thing. Coaching means telling them what they're doing right and pointing out where they can improve. Be constructive and critical. Don't be negative.