Working both ends of the problem
Clearly I'm not an expert (as I shall now reveal with this question) but, as well as trying to mitigate the amount of damaging radiation that hits you in the first place - with lead, water, "force fields", etc. - what are the prospects for being able to just fix the damage?
Medical nano technology clearly isn't there yet, but is it within in the realms of plausibility (in the same kind of timescales that we are talking about for travelling to Mars, etc.) to imagine that you could have nanobots roaming within you that could just fix up a reasonable amount of the radiation damage or at least cleanly dispose of newly formed cancerous cells, etc.?