What are the avenues to significantly decrease data centre energy consumption?

Data centres consume large amounts of energy, both for power and for cooling/air-conditioning. As computing becomes even more pervasive worldwide, data centres and the amount of energy they use will increase even further. Making data centres more energy efficient hence will contribute significantly to a more sustainable planet. While the large tech companies have been working for quite some time on making their data centers quite efficient, innovations can result in even more energy efficient data centers. What could such innovations be?