Eric Hedman has some ideas. I agree with Clark Lindsey--he is far too optimistic about the prospects for scramjets providing a solution to the problem:

Scramjet propulsion theoretically has the advantage of increasing a vehicle’s mass fraction for payload by reducing the oxidizer load it has to carry. The big question is, can this theoretical advantage be turned into a practical advantage with a solid, sustained research and development effort? I think it’s time we find out.

The answer to his big question is...probably not. While the military may have applications for scramjets, I still think that it would be a foolish priority for technologies to reduce launch costs.