Liszt was a flamboyant performer: one of the first to improve the recital as spectacle by turning his piano sideways, he'd played in green silk gloves, and later leave them for the swooning, jewelry-tossing ladies in the audience to fight over. He advised his students to read a book while practicing.
The advantage of reading a book while practicing for pure technique alone is that it enables us to forget the boredom of playing a passage over and over again, a dozen, or fifty or a hundred times until the body has absorbed it. Not all books, however, lend themselves equally well to this employment. Poetry interferes subtly with the rhythm of the music, and so does really admirable prose. The most useful, I have found for myself, are detective stories, sociology and literary criticism. However, any reading matter that distracts the mind without engaging the sense or the emotions too powerfully will work.
[From Piano Notes by Charles Rosen. This via Phil Gyford, thanks, who has his own working trick.]
Liszt apparently read Shakespeare and Goethe (among others) whilst practicing.
With this exercise, he went through all twenty-four keys and relative keys; he did the same for the arpeggiated octaves. ... He does all this for hours on end, while at the same time reading to avoid boredom. This is the time, as he exercises his fingers, that he meditates over his readings [...] He therefore insists that each finger should be exercised for a quarter of an hour daily by raising it very high and releasing it downwards, not on its tip, but instead upon the ball of the finger. This exercise might be done while reading in order not to become bored.
[Boissier's A Diary of Franz Liszt as Teacher 1831-32, trs Elyse Mach, p23]
Others: Henselt reportedly read the Bible while practising exercises, which he did for up to sixteen hours a day - when he got to the end, he would simply go back to the beginning and start all over again. Saint-Saens did his piano practice while reading the daily newspapers! Brahms. But this advice isn't supported by all teachers.
More How we work.