Most metal singers have at least some training or have taken vocal training. Training will increase your ability to sing in general. Learning how to breath properly and use your diaphragm properly can make your vocals stronger and increase your singing stamina.
Learn to properly use your microphone to help you with your dynamics, which involves the volume of your voice and how close or far away you will need to hold the microphone for consistency. Most metal singers do a lot of screaming as well as singing, so you will need to know how to hold the microphone properly for each individual method of singing. Many trained singers pull their microphone farther from their mouth when they are singing loud and closer for softer passages making the vocals seem very even in volume.
Drink warm liquids before rehearsing or singing metal songs. Warm liquids tend to relax the throat and open up the vocals cords to sing in many different styles. Warm tea or warm water can also be used between songs if your throat gets sore or scratchy. Rest your voice after nights on the microphone by limiting conversations and getting as much sleep as possible.
Warm up before singing to get your vocal cords ready for a night of metal music. Metal tends to be harsher on your vocal cords than other styles, depending on the type of metal vocals employed. Death metal and thrash are very gruff and can cause your vocal cords to be sore, so sing one song a week in this style for a few weeks and build up to two songs a week after you feel comfortable. Lighter metal singers should concentrate on improving their range by pushing their voice to the limit and reaching for higher and higher notes every week. Enlist a vocal coach who is familiar with the training necessary for metal vocals