When singing, always take deep breaths and sing from the diaphram. Remember your vocal chords are small muscles and will fail you if you over work them. Its almost like pushing to take a crap, only directing all that wind power over your vocal chords. You should never see the vein in your neck when you are going after a high note. If you do, you are doing it wrong and can severely damage your voice, not to mention going hoarse during your gig. If done correctly you should have just as much voice at the end of your gig as you did when you started.