Try looking for any available foot placements outside the crack also, keeping 1 foot tip smeared in the thin crack and the other on an edge. If there isn't anything, you just have to stick your foot in sideways and weight it. Keep your heel just low enough to allow more contact with the sole of your shoe.
Heel slightly lower than toe Submitted By: Lorenzo Tragen on May 5, 2013
The real secret to finger cracks is upper body strength. Living in the desert and climbing cracks all the time I've learned that it doesn't matter how good your technique is when it comes to finger cracks if your not strong enough to lock off on one arm. Everyone you see hiking finger cracks are really strong and thats why their footwork looks so bomber. That doesn't mean that some good footwork won't help, it just isn't going to be the limiting factor. On a splitter crack with nothing for your feet I would estimate that your only getting about 10% to 15% max assistance out of your feet, the rest is upper body. Also keeping your feet high on splitter finger cracks is key so they don't keep skating.