I started attempting vocals about 6 years ago when I first got into extreme metal (black, death, etc), it took me attempts off and on through these years to be able to do a comfortable vocals.
Vocal styles I always liked the best and tried to imitate were Carpathian Forest, Carcass, At the Gates, Black dahlia murder (highs) etc.
The best way to do it Painlessly and healthy is not to put strain on your vocal chords. When you try to do any type of extreme vocals you can feel where the "vibrations" that make your voice distorted are comming from.
The best tip I have for this without getting scientific about it is to keep these vibrations at the bottom of your throat. Control it to where you can keep the vibrations for the vocals your trying to do from making your throat shake. Sure, you can get some evil sounding stuff straining your vocal chords (and get them alot easyer), but its basically tearing your vocal chords up which isnt good. If you dont know what im talking about, put your hand on your throat when you try to do it, if you can feel your throat vibrate then your not doing them right.
Do not try to make yourself REAL loud, thats what a mic is for. You can get away with doing quite vocals if you have a good enough PA setup. Sure, it will help if you can sing louder, but it will take time a practice to be able to keep it loud enough.
If you feel like your getting a cold, chances are your doing it wrong and need to try to quit using your throat for these vocals. Move it down in your throat you should be able to feel it. I remember i tried to do Nattefrost vocals for like a week and strained my throat all to hell, and i got a head cold for about 2 weeks after that. That was a major mistake and if a definite sign that I was doing it all wrong. Try being in a band that plays shows almost every weekend or tours - do you think the other members of your band want to take a 2 week break because you cant do vocals right and are basically destroying your voice?
The thing is though, it took me up until about a year ago to do vocals for a decent length without vibrating my vocal chords so much and to get a good sound from them. It takes alot practice to get a good sound without straining your throat.
Drinking water is the best thing I have found to help aid this process, if you feel anything getting sore, chug some water and rest for a few seconds. Over time you will build up the ability to do this without resting as much.
Smoking cigs (although I do smoke) IS NOT good in any way for you. There are plenty of good death/black metal singers who havent smoked in their life. It doesnt help you, it will probably only give you throat cancer which equals you using a voice box.
If this doesnt help you at all, then maybe your just not made to do this style of vocals. But it seems to be working for me.
I will try to put some samples of my vocals up soon once I figure out where I can upload them so you can get an idea of what I can do.