From my experience speaking to our clients in HR department, I believe it is most important to give employees a voice.
The key is for organizations to treat employees as their most valued resource.
This involves investing in areas such as professional development and workplace wellness. But even more importantly, employees need the opportunity to provide input and receive meaningful feedback from leadership.
More broadly, employee engagement requires a genuine commitment to continuous dialogue and organizational flexibility. Depending on what an employee needs – the behaviours, processes, and ways of doing things may need to change.
The best way to start is to ask employees the right questions about their work. Web-based technology has made it easier to involve employees and manage the data that is gathered so that organizations get the answers they are looking

Omri Mor’s answer to What is the single most meaningful initiative that can be taken in order to improve employee engagement? - Quora