I'm not sure that muscle memory is the right concept here.
Muscle memory is certainly important: for example, it's important that your flicks are well calibrated (so that you always apply the right amount of force to move the mouse the precise amount needed).
What you experienced was what some term peak performance.
Typically experienced as effortless, on fire, in the zone, etc.
Reflexes improve, and time slows down. You are a calm, zen, warrior, and there is no anxiety or ego.
If you are in this state and you lose, you feel like congratulating your opponent, because you realize she has earned the victory against your best self.
Many people use drugs to achieve these states, others are naturally good at them.
There are probably a few things you can do to actively influence the probability of achieving such a state, and you might want to research techniques that athletes use (gymnastics, figure skating, and billiard-type games, tennis, etc.).
Your physiological state is what counts here, both of the body and the brain, and it's probably important to find which states work for you, and try to achieve them consistently.
For example, you may perform best during certain "bioryhthmic" states. Having a consistent sleep schedule, and playing at the same time, will allow you to develop your peak-performance-cultivation skills within a more consistent physiological environment.
Nutrition and digestive considerations should also not be ignored. I find I am most relaxed after a good bowel movement, so when I work out, cycle, or play volleyball, I try to ensure I've had a decent crap
Meditation may also help. I haven't experimented with this much, though if I start playing again I certainly will. Certain forms of meditation are great for reducing the workload of the brain, so as to free up mental resources for the task at hand. Breathing exercises combined with a conscious effort to eliminate thought is a good exercise.
It also follows that on average, you will probably perform better if you have no issues digging away at you. Try to resolve issues as much as possible before trying to attain peak state. Else your mind will be working to resolve them subconsciously (which is probably experienced as background anxiety). As an example, you probably want to have finished your essay that's due the next day, if you want to perform well in quake.
Visualization may also help. Try to visualize those crazy multi-axis flick mid-air rockets in advance.
Ideally, you want to find your own preparation routine which you can use before you start playing. Czm told me that he used to listen to a certain song before playing important matches. Everyone has their own "song", and that song may lose its potency after time.
The most important thing is that you learn to
recognize the state of mind when you're in it, and become mindful of it.
It's the same principle used in bio-feedback. People can learn to control their heart rate and brain wave activity if they are shown a monitor which gives them feedback about their heart/brain states. I've heard one person describe their ability to change their heart rate as putting your hand in a black box and pulling some unknown lever - they're not exactly sure how they do it, or at least they cannot verbally articulate it, but with training and experience, they can do it more-or-less consistently at will.
Another thing - if you find yourself out of the zone, don't be afraid to just accept that today's not your day.
If you're in a real rut, it may mean that you've become saturated with detrimental habits, and need a break.