In singing, see that the neck and head are in a free and loose condition. If the voice is focussed forward as evidenced by pure and beautiful tones, the breath will economize itself. Mange the voice with the ear. Don't manage or try to manage the vocal mechanism. When the muscles in the neck are in evidence during song, it is a positive indication that the tone is not well produced. When the corners of the mouth are drawn back, the tone will become hard and strident. Mouth or facial muscles should not be rigid or set. Never smile artificially. The smiling countenance representing joyousness must be a natural result of happy thought. It is spiritual in its origin.
In general, the singing voice of the Italian is characterized as open or closed. "Piu aperto" or "Piu chiuso" representing their preference. The consensus of opinion, however, is that "Piu Canta, Chiuso Canta diesci anni di piu" (He who sings closed sings tens years longer). The import is that the use of the voice well focussed forward is more conducive to its long life.
Students should realize the importance of doing small things well. If this is attended to, the big things will take care of themselves. Always sing exercises with strict attention to tone quality. In regard to all graded exercises—so-called methods—always remember that the important consideration is not what you sing, but how you sing it.
Forcing the voice is generally the result of either the desire to sing too loudly or with too big a tone, or of attempting to locally adjust the vocal mechanism; in short, singing with the throat instead of through the throat.
The natural position of the voice in effect, unhampered by rigidity of the body and sympathetic rigidity of of the throat, is forward in the mouth. Correct forward placing is never forced.
—The Lost Vocal Art and Its Restoration by W. Warren Shaw: 197-198. Find this text on VoiceTalk's download page. It's one of the really good (and forgotten) books on the Old Italian School of singing.