Some basic thoughts on the fizzix of shields..
You'd need different techs for the various things you'd want to shield against.
For heat shields, ablatives are good. These are generally fairly low-density materials designed such that they insulate, while sacrificing their outer layers which are blasted and/or vapourised away, removing the excess heat with the shed matter. The best of these by far has to be a special type of plastic dubbed Starlite, created by an Englishman named Maurice Ward. The truly incredible properties of this material has been attributed (by Jane's International Defence review) to a 'reactive armour' plasma type effect; being entirely passive, the efficacy of the shield seems to depend only on the amount of energy thrown at it - the more you heat it, the more heat-resistant it gets.
This brings us to another good type of shield, then, plasma - however it's generated. Besides dealing with heat, a plasma could also ionise particulate matter. One popularly-cited form of shielding for high speed space flight is based on a large plasma bubble, contained by magnetic fields emanating from the ship, and extending some distance in front of it - perhaps some kilometers. This could effectively turn problematic high-velocity dust grains into relatively harmless puffs of smoke, before they strike the ship's hull, not unlike an icebreaker clearing its own path through the pack ice.
Beyond these types of environmental protection, a magnetically-contained plasma can also shield against some types of weapons fire - specifically, small projectiles (as described above, if exaggerated slightly), as well as plasma bolts, and also laser fire - this by modulating the plasma's pulse rate to offset the phase of the incoming beam, effectively cancelling its energy.
So, bashically, plasmas, and their control systems, are a handy catch-all technology that could defend against a broad variety of insults...
As others have mentioned though, sub-atomic KE, AKA radioactivity, is harder to protect against... what you really need is high density material, to increase the chances of high-velocity ions hitting a shield particle before they hit you or your hardware. Lead is obviously the de facto no. 1 choice, being very dense and stable. Water has been mooted as a more practical alternative, making up for its lower density with its greater utility in regards to life support systems and so forth. However this is one type of hazard that plasmas would seem fairly useless for, owing to their low density.
So in short, plasmas are a good general purpose shield, and ditto materials that form plasmas when assaulted. But radiation protection really needs a high-density barrier, to filter out the sub-atomic buzzbombs that'd breeze through thinner materials.
Lastly, it has to be admitted that relativistic speeds (and remember 26c and higher is already confirmed for cruise mode) will basically turn the ambient background radiation into super-killer alpha particles from hell. The net radiation from all stars in the sky will be shifted up into a maelstrom of cosmic rays, it'd be like bathing in a constant beam of gamma ray bursts.. You'd be fried in seconds. The half-lives of any unstable environmental elements will shrink in relation to you and your ship, and anything even slightly radioactive will seem hotter than plutonium. I'm not aware of any ideas to tackle this last hazard, and mile-thick lead hulls probably aren't very practical.. probably best to just not think about it?
