this might sound surprising, but it's all in the thumb. Start in the air with totally flat fingers and a cocked, bent thumb tucked alongside the 2nd. Extend the thumb straight out as far as it goes and then out to the side (slightly down, so as to open a massive arch between 1 and 2). As you do this, notice the fingers. They will likely draw back towards you from the knuckles. Practise this movement in various different ways- sometimes deliberately pulling back the fingers and sometimes letting it happen passively. As you do it more, start paying attention to the 5th knuckle instead of the 2nd. Let the 2nd start to just follow and only notice the thumb and 5th consciously. The thumb goes forward and out while the fifth pulls slightly back, causing the knuckle to be primed.
Gradually, you should get to the point where the knuckle of the fifth forms its position even if there is no conscious effort at all from the finger (and only a very small one in the thumb). It's purely a very low effort reflex reaction in response to the thumb. I'm going to write this up in more detail on my blog at some point and illustrate with videos. This has completely transformed the shape of my right hand. You can fix a sagging knuckle/fingers that fly up very easily. I still catch my fifth doing this sometimes, but it's always paying attention to my thumb that fixes it the easiest. Trying to fight it in the fifth itself just causes fixations.
Topic: A rebel pinky finger