The basics have already been addressed. Livable pay and manageable hours. Do not underestimate the benefits of streamlining your chores. As someone else said, a large capital investment (like a gator or water lines) will not only save you money in labor, but will reduce your turnover.
But get to know your employees and what makes them tick.
I have one amazing employee who is a mom and brings her daughter to work some days when school isn’t in session. Her daughter has been to horse camp, is safe and well behaved, and even helps with chores. I have no problem with that.
Other employees really want to learn about all aspects of horse care, so I make sure I’m teaching them and not just doing it myself. I always offer for them to come watch and talk to the vet or watch me or another trainer do something (like ground driving) so they can expand their knowledge.
A lot of people like barn jobs because it’s a somewhat/mostly solo job, but it can get lonely. Take the time to stop in and chat with them. Take a real interest in their lives and follow up with “how was Joey’s game on Sunday?” People like to work for people who care.
Mix up the tasks. 6 hours of mucking is a lot. 6 hours of feeding, turnout, mucking, waters, and dragging the arena mixes things up a bit. Not only is it good for the brain, but it’s good for the body.
Lead by example. Make sure you’re spending time in the barn helping with chores. Obviously it’s not possible daily, but don’t be “above” mucking. Beyond that, make sure they actually see you doing it.
Do whatever it takes to make their job easier. Is it ridiculously cold? “Sue, go sit inside for 15 minutes and warm up. I’ll do this stall for you. There’s coffee/tea/hot chocolate if you’d like it.” Unbelievably hot? Offer cold water/Gatorade a few times during the day.
That last piece is really important. In a lot of ways, my employees are also my customers. I’m responsible for making sure they want to work here.