he student who wishes to aquire a firm technique on the guitar should not neglect the patient study of scales. If he practices them two hours a day, he will correct faulty hand positions, gradually increase the strength of the fingers, and prepare the joints for later speed studies. Thanks to the independence and elasticity which the fingers develop through the study of scales, the student will soon acquire a quality which is very difficult to gain later: physical beauty of sound. I say physical, because sonority and its infinite shadings are not the result of stubborn will power but spring from the innate excellence of the spirit.
In order to derive the greatest possible benefit from the following exercises, play them slowly and vigorously at first, more lightly and rapidly later. In one hour of scales may be condensed many hours of arduous exercises which are frequently futile. The practice of scales enables one to solve a greater number of technical problems in a shorter time than any study of any other exercise.
-Andres Segovia
Please also see the Andres Segovia Guitar Transcriptions.