Well as has been said a couple of times, muscle memory is the key.  Play long enough and certain changes become subconscious.  Improvising lead and singing is another matter, I've never personally been able to do that with much success personally although there are people who can.  If you're just strumming along or playing a particular progression, just practice it over and over and before you know it you'll be able to sing without even thinking about the changes.  Good luck.

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