[jadyoyster]

Heat shielding is needed, as well as extra landing engines that work outside of vacuum, since the second stage engine would destroy itself under atmospheric pressure. It's easy to see how quickly you start to get dimishing returns, but it would be cool if they could pull it off.

[avmich]

Some experiments were done in early 2000 with inflatable aerobrake. The concept was shown to work, but experiments weren't a complete success. However this technology I'm sure can be applied to the task of returning the second stage.

https://en.wikipedia.org/wiki/Atmospheric_entry#Inflatable_h...

[raducu]

Or even more futuristic, plasma parachute/heat shield: http://www.nasa.gov/offices/oct/early_stage_innovation/niac/...