This will sound really silly, but for me, I keep mine in my left pocket, and my habit is to put it in as it is held in my hand when I'm using it - i.e. screen toward leg and top of phone towards ground. Well however I move, sit, walk, whatever, kept pushing it open ever so slightly and having that exact problem. So all I do now is try and remember to put it in my pocket the opposite way - the opposite of however my body is naturally pushing it. So now I make sure and put it in screen out, bottom towards ground.
Like I said, silly, but I haven't had the problem any more. Of course, that's all based on the presumption that the same body movements are causing the problem each time! How's that for some over-analysis of the situation. :icon_eek: