What are the concepts, solutions and technologies to store electricity at low costs?

The Achilles Heel (or the Holy Grail, if you wish!) for renewable power is energy storage. If only we can store renewable power on a large scale, the key challenge of intermittency in generation of renewable energy sources such as solar and wind is taken care of. But storing energy, especially electricity, is still very costly. What are the avenues by which we can bring down the cost of electricity storage significantly?