Like anyone involved in regular physical activity, it is important for a singer to keep fit. All the muscles, ligaments, tendons, etc. used in singing and stage movement require regular exercise to keep them flexible and in the best condition for performing.
It is also important not to strain your voice, as this can lead to poor technique along with a lessening of the sound quality of your voice. In extreme cases such as repeated shouting, this can lead to vocal nodules. These are growths on the vocal cords which can cause permanent damage.
Nodules usually occur in Rock singers who sing with a rough technique at high volume (shouting) over extended periods, often in an attempt to compete with the volume of electric guitars played through large amplifiers.
If you are singing with a band, it is essential to have adequate foldback. Foldback speakers are ones which are aimed at the performers rather than the audience. They are there so the performers can hear themselves properly when amplified through a PA system.
If you are having trouble hearing yourself, you should never sing louder to try to overcome the situation. Instead, you will either need to turn the foldback up or get the instrumentalists to turn their amplifiers down, or both. Many beginning electric guitarists and drummers are not used to playing at quieter volumes, but it is essential that they learn. Otherwise you may end up with vocal damage.
When rehearsing songs, don’t forget that dynamics are an essential part of music. If you always play and sing too loud, you have nowhere to go. Get the musicians accompanying you to play at a comfortable "middle" volume and work out the best places in the song to increase or decrease the volume for dramatic effect.
When performing, everything you do either as a singer or an ensemble should have the sole purpose of communicating the song to the audience. This means moving them emotionally rather than deafening them.
Another aspect of looking after your voice is drinking plenty of water. Other liquids can adversely affect the sound of your voice, but water keeps the throat and larynx lubricated without affecting the sound.
It also has the added bonus of preventing dehydration if you are using a lot of energy on stage. As well as this, a sensible diet and regular exercise will help keep your body in the best possible condition for singing and performing.