Pick one foot that you unclip at every occasion and stick with it. I go with the left because it's usually comfortable to rest it on a kerb if required etc. Then there's no doubt in your mind approaching the junction regarding what exactly you're going to do and there's less likely to be panic.
It is also key for your balance. There's nothing worse than arriving at the lights with a left foot unclipped, then realising that your weight's on the right and you have to perform an emergency unclip with the other foot while your left isn't secure in the pedal.
It will soon become second nature though. FWIW, unclipping is a very rare issue for me, but I still have occasional issues getting clipped in, especially when trying to take off from a junction quickly or when carrying a lot of weight on the bike. The key to this is to just take your time. You're never holding anyone up as much as you think you are.
Good luck
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