Record real violins.
Seriously, there's a lot of tweaking you can do to try to make it sound less fake, but the fact remains that the real thing is capable of being played with an infinite continuum of variations -- which is what makes it sound real -- while no matter how good of a set of samples you have, they are still samples and fiddling with them (no pun intended) is not the same as what a violinist does to his instrument. You can't pluck the sample or bounce the bow off it, you can only modify MIDI parameters... it just is not the same. You'll kill yourself trying to make them sound truly real.