Buzz Aldrin's Cyclers
Apollo 11 astronaut and MIT PhD Buzz Aldrin proposed just such a concept several years ago. His idea is for a large cycler spacecraft that would fly between the Earth and Mars without stopping to enter orbit at each end. Instead of a mailbag snatch, spacecraft would fly out from the Earth to upload new crew and supplies, then slow down to return to the Earth. Similar spacecraft would do the same thing at the Mars end. You'd still need a fair amount of energy to do these maneuvers, but since the transfer vehicles would be much smaller than one needed to go all the way to Mars, it would require a lot less energy.
The devil is in the details of the cycler orbit, as explained in the article. There's a lot of info on Aldrin's cyclers on the web.