Now before you even attempt anything on this page, make sure you have been screaming for at least a couple months and have a good idea of how to do all the basics for your scream.
Improvement 1: So now you have a basic low. It's pretty decent right? Well how would you like to sound like Phil Bozeman (Vocalist of Whitechapel)? I found out his technique not too long ago actually. So what you do is take your tongue and curl it to the roof of your mouth and lift your soft palate. Now if you don't know what your soft palate is, it's a soft patch of skin on the very back roof of your mouth. You can feel it raise when you yawn. So go ahead and do a fake yawn. Now that you know how to lift it, just curl your tongue and do a low and sound like Whitechapel!
Improvement 2: Okay, so maybe you don't like Phil's lows. So how about Mitch Lucker's (Recently deceased vocalist of Suicide Silence)? His are pretty simple as well, but just as brutal in their own way! So basically what you do is combine your mid scream, and your low. That's all there is to it! Just do the lowest mid you can do without actually making it a low. At first it might not be as low as you want, but that comes with practice my friend.
Improvement 3: Okay, maybe you don't like Phil's or Mitch's lows. This is how I personally do my lows. If you can whistle then you can do my lows! So go ahead a whistle. Now hold that throat shape and just do a low. Note your throat shape will change slightly to accommodate the scream and that's completely fine!
To be honest I haven't really found out much with mids. The only thing I can advise you to do is add more of your voice into it. That usually gives it a more full sound and unique sound. Just try it once and you'll be amazed by how much your scream just changed.
Again, I don't have much to say about highs because my main focus are my lows. So I really haven't dabbled into finding more ways to improve highs or even mids. Though I am always looking to improve so I'm sure this will be updated sooner or later.
So basically you take your regular old high you have, and then just add a natural yell to it. This should make your scream more smooth and slightly higher. With time you be able to achieve extremely high screams. This video toward the top gives an example of what I am saying, so if you haven't already go check it out.
How to do Mitch Lucker's Screams
More "improvements" coming soon.