How to practice SPD release in very steep uphills? Some time ago I got SPD pedals for my MTB.

When going downhill or rather flat these pedals are awesome. However, I have problems using them on sharp uphill with roots/rocks. Because it's steep, I need to use a lot of power but suddenly the bike might just stop in a heartbeat, since the back wheel might slip on a wet stone/root or something. Currently I usually crash at that moment with SPD pedals but with flat pedals it's no problem. 

The question is: How do I practice to get really good at stepping out of the pedals at very steep uphills, without actually trying and crashing (since it hurts alot!)?
Practice, practice, practice... You need to move the physical motions of getting out of the pedals from your conscious muscle memory to your unconscious muscle memory. Once it becomes an instinctual unconscious reaction, you'll have far fewer problems. 

A flat grassy space is good for this. Try doing track stands and un-clip to catch your balance. 

Or you can just ride and eventually it will become second nature. The more you practice, the quicker that will happen. I don't ever think about unclipping, it just happens when I want it to. A lot like the actions of driving a car, do it enough and you don't have to think about it. I can't remember how long it took me to get there, but it will happen eventually. 

The one gotcha that still remains is that it can be very difficult to unclip when your weight is still on the pedal. You can get good, but there will still be times
where clipless pedals aren't as escapable as flat pedals.