Radiation Shield

The limited Martian atmosphere poses a number of problems for the would-be human Mars explorer. While dust storms and clouds may do enough to deprive the MER solar panels of solar radiation, the overall atmospheric coverage (of things we can and cannot see) compared to that of the earth is barely noticeable at best.
A human on the surface of Mars (let alone in space for the duration of the trip to Mars) would be exposed to a significant amount of radiation which becomes a problem after more than a few hours exposure and would certainly be fatal in the months or years spent on the planet. The Apollo astronauts who spent hours or over a day on the moon were "lucky" in that there was little solar activity at the time of their missions, and their relatively short stay on the surface meant that there was little risk.
The sun continuously radiates in varying amounts across the EM spectrum, and in the case of a solar flare there is significantly more radiation which can be dangerous to anyone outside of the earth's protective atmosphere which filters out most of the dangerous radiation.
An article posted at http://www.space.com/businesstechnology/lunarshield_techwed_050112.html
explains some of the ideas for radiation protection on a moon base. The theory is that if electric fields are created around the perimeter of the base, they will effectively create a sort of force field to block out radiation from the sun. There are a few problems with this, such as the effect of the field on the base itself, the optimal hight of the field generator towers, what energies would be filtered, etc., but this is so far one of the most developed theories they have for addressing the issue of radiation.
The article itself talks about using this field generator on the moon, but the same idea could be applied to a base on Mars.