The health of your voice has a lot to do with where you sing FROM. A lot of people have problems after extended singing because they produce their sound from the shallow part of their breathing apparatus. Always produce sound from the depths of your diaphram. This will give you a more powerful voice AND it will last much longer. Also, over-tiredness is a killer. No matter how many gigs, how much travelling you do, make sure you get enough sleep - if you can!