I've recently come across an interesting way to practice. I was asking a very accomplished guitarist on how he practiced technique, since his was amazingly effortless.
What he told me surprised me. He said he'd watch hours of movies etc. while having his guitar in his hands and running through different permutations of fingers. The most important part here is that he claimed to not really pay any attention to what he was doing consciously and also that it was the core of his technique practice.
Now, I wouldn't want to limit this question to guitar, or even music, because it is very general. Therefore: how much motor skills can you learn and how much muscle memory can you acquire while simply repeating the act over and over without paying attention to it? I'd love to hear what experience you've got with this, thanks!