I like to keep the corners in. When I slur up I move the back of my tongue upward. I have read on here from professional players not to do this but it works for me. After you do this for a while you will find that you don't have to think about it and that action won't be as drastic. On TH. there was a big debate about this. The thing that I remember reading that contradicts my approach is: "what do you do when you start really high." I think it is just an exercise to practice until you get it.