Does anybody have any tips for me for learning how to scream?

So I'm learning how to scream metalcore/post hardcore/screamo, however you put it. I'm decent but I can see at points where I'm horrible, like clean screams. My high pitches are pretty good and my low pitches are getting better but when I try to just do a clean scream, I fail. Tips? Also, I don't want to do just clean screams so if any other tips on any other type of scream, I will appreciate it! Thanks! ^-^