Through years of trial and error - skipping around internet radio stations, playing our entire music collections on shuffle, or just hammering single albums on repeat, we have found that the most effective music to aid prolonged periods of intense concentration tends to have a mixture of the following qualities:
Textures without rhythm
Minor complex chords
Early music (Baroque, lute, harpsichord)
Very few drums or vocals
Walls of reverb
Music possessing these qualities can often provide just the right amount of interest to occupy the parts of your brain that would otherwise be left free to wander and lead to distraction during your work.