When herding cats, it is better to carry a can opener than a cattle prod.

While this is difficult for traditional management styles to grasp, it is most useful to just ignore the down times and instead do absolutely everything in your power to encourage the ultra-productive "flow" state to happen as often as possible. The necessary conditions will vary by developer. Some people thrive with regular teammate feedback. Others like to pull a lonely all-nighter. Some will do their best thinking on a treadmill or in the shower, then sit down and execute. Some perform work-related rituals, like the morning coffee ceremony.

There is no single thing you can do, other than getting to know your co-workers. Once you know them well enough, it should be obvious what you need to do. It may well be that the best way to motivate one of your developers is to institute timelines, milestones, and deadlines, but another will just switch off the creativity and motivation because of the micromanagement, and they won't come back until well after you remove the artificial pressure.

There's no magic bullet. Sorry.