Obama’s new potential budgetary influx to NASA has re-opened avenues of research. One of them is the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) technology, a privately-funded resurrection of plasma rocket prototypes from the late ’90s that could trim travel time to Mars down to just more than a month.
The problem is power.
In order to reach those kinds of travel times, a power source nearly 1,000 times more powerful than what is currently available on-orbit is required. It looks like our next breakthrough will need to involve orbital power generation.
Something to think about…
Chronicles of an Atomic Astrowright 
Nuclear is the only sustainable way, but even with high temperature operation exceeding 40% efficiency, 60% will go into waste heat to radiate. Just if there’s a material that will convert waste heat to electricity and weights feather light and has a high efficiency… Oh future…