I would practice like this:
Find something you can balance the bike with, so you aren't in motion... You can't really clip-in unless you put your full weight on it. The important part is knowing the motion you need to do to release from it.
It's a twisting motion, twist your heel away from your frame. After you do it with each foot successfully while stationary, then try riding around an empty lot.
Always use your dominant foot when you stop. As you prepare to stop, stand on your non-dominant foot and unclip your dominant foot. Make sure your balance favors a landing on that foot, not the opposite side. Always using the same foot helps ensure this.
Always give yourself time to do this. You don't want to go down at a traffic light. As pointed out above, it's when you get surprised that your most likely to forget to unclip.
I've fallen a bunch of times, exactly because of that. Fortunately, it's usually when you're trying to stop anyway, so you probably won't get hurt.
Don't unclip your other foot until you stop, especially going from traffic light to traffic light... Use the still-clipped-in foot to pull the pedal up to a good take off position. If the distance to the next stop is short, don't bother clipping it back in.
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