First things first - remember the old saying: "People join a company, but they leave their managers". Most likely, your management is failing your employees. They are not providing the right work, the right career growth, the right sort of working environment for them. Individuals are different, so there will not be a catch-all solution for retention.
What can we do in order to retain the good people?
By satisfying their needs from their job. The common refrain for motivating/engaging people is three parts (popularized by the book "Drive: The Surprising Truth About What Motivates Us"):
Are you giving your best and brightest the freedom to do what they need to do, without hassle or headache? The old-west of startups provide a lot of freedom, compared to bigger companies. But even as a bigger company, there's a difference between necessary interaction to work with the larger pool of resources and overhead caused by this unweildy beast.
Are you letting them learn and grow their areas of expertise? Startups tend to be challenging environments, pushing the boundaries of employees' skills. Big companies can get complacent - trying to maintain their status rather than pushing the boundaries.
Do your employees understand what they need to do, and how that helps the company go? When there's only a dozen employees, it's really clear about how your contributions help; it's really clear what needs to be done, because you see what everyone else is doing. In larger companies, it's not readily apparent. Without good communication (often via 1 on 1 meetings with their manager), employees can lose sight of that, along with there sense of purpose in the workplace.
But again, different employees have different needs. Your management should be working with employees constantly to determine those needs and address them individually. Some will want more responsibility. Some will want more training and learning opportunities. Some will want different/more challenging work. And it takes skilled leaders to fish out those desires from people who rarely realize consciously what makes them happy.