How do you sing like Mikael Akerfeldt from Opeth?

You've gotta growl.

Rather than using your throat to growl, its mostly in the diaphragm. You push hard with your diaphragm, and make sure you use the proper technique with your larynx. Try putting your hand on your Adam's apple when attempting this. If it's moving up by a fair amount when you're attempting your growls, you're on the right track.

I just got the hang of it recently, and using technique like this, I've gone from only being able to scream and growl for one song per set to being able to growl for hours without any pain.

Also make sure and check out different resources online, and try and get a copy of Melissa Cross' Zen of Screaming DVD. Everything you learn about these things helps.