Scream: No matter what you've been singing or how, turn the last word into a scream and get everyone's attention focused right back on you. Make that last syllable go on and on with a growl that turns into a shriek.
Trill and have a crazy sick falsetto. After screaming, the other attention-grabber and required singing skill of the heavy metal singer is the insanely high voice that sings an actual melody. There are many times a song requires a bridge or a small piece of music that is actually sung. It's usually high falsetto -- and get that falsetto trilling.
Actually sing well. No matter what you want to project, or what your chosen kind of music, you still have to be able to carry a tune, in tune and with a decent sense of rhythm. Make sure you have the ability to sing and the charisma to carry off the rest.