Environment is tops in the retention of people. There are two good examples. Let employees experiment with new toys. Technical people want the opportunity to learn and try new things. Training can be a double-edged sword.
Training allows current employees to obtain new skills then get new jobs and leave.
But training is important. They are unappealing and no one wants to work like that. It takes away from the collaborative effort. These are technically skilled people, but they are also very creative people and need the brainstorming opportunities. Write a good description of the job.