You just need more mass. Have a look at this concept: http://eduard-heindl.de/energy-storage/index-e.html (unfortunately the english version is very sparse, the german version has some more images and videos: http://eduard-heindl.de/energy-storage/ )

The idea is to cut a 1km diameter cylinder into the ground (solid rock), and then lift it up up to 500m by pumping water underneath it. The power stored can power a country for a day.

I am somewhat sceptical, but this guy is totally serious about it.