Being in the zone (lots of energy, ideas, inspiration, optimism, focus) is really a vague description for when your body is functioning optimally.

People who are depressed are given SSRIs (selective seratonin reuptake inhibitors) because your mood is largely governed by the levels of seratonin available in your brain. Similarly, your focus / "motivation" / "drive" is largely determined by dopamine. Acetylcholine, another neurotransmitter, is described by Wikipedia as having the key function of "responsiveness to sensory stimuli, a form of attention". The neurotransmitter GABA is commonly referred to as the "relaxatory neurotransmitter" - the one released when you do yoga or meditate.

The levels of these neurotransmitters are affected by what you're eating (and your body's ability to process what you're eating in terms of your baseline metabolic rate, digestive ecology etc.), how much exercise you're getting, what you're doing and even the thoughts you're having themselves.

So, in my opinion at least, you could equate "being in the zone" with having all your neurotransmitter levels and other body mechanisms functioning at their "optimal" levels. What complicates this is that "optimal" is person-specific. So a person who is "out of the zone" and very lethargic could make themselves more "in the zone" by taking steps to raise their dopamine levels whereas someone who is "out of the zone" because they're too over-active could become more "in the zone" by raising their GABA levels and bringing more "calm" to their life.

Raising your levels of a particular neurotransmitter can be done directly or indirectly. For example, you can raise your levels of the relaxatory neurotransmitter GABA by drinking green tea which is high in theanine which is the amino acid which your brain uses to make more GABA. Alternatively, and more interestingly to me, is that your actions affect your neurotransmitter levels too. For example, see the link below to a study concluding that a "12-week yoga intervention was associated with greater improvements in mood and anxiety than a metabolically matched walking exercise. This is the first study to demonstrate that increased thalamic GABA levels are associated with improved mood and decreased anxiety."
http://www.ncbi.nlm.nih.gov/pubmed/20722471

I can't really substantiate this and it might very well sound like hippie bullshit but I honestly feel that there are ways to live your life that are better than others. For example, I think it would be difficult to argue that you feel better if you spend the day outside than if you stay inside sitting at a desk in an air-conditioned office. Of course, you can point to increased vitamin D being correlated to improved mood etc. but I think we all have an instinct that certain activities are "good" for you (in the sense that they help you become more "in the zone").

I've found the following to help move me more in alignment of "being in the zone" and move my neurotransmitter levels in the directions of "where they should be" for me to be in the zone:
* 15 minutes of yoga every morning

* Omega 3, vitamin D and a solid multivitamin every day

* 40 minutes of intense cardio 3x per week (7-10km run)

* compound functional weights training (squats, deadlifts, pullups, etc.) for 40min 3x per week

* cut out sodas and drink at least 2.5 litres of water daily

* try not eat heavy meals at night (your digestion goes to sleep when the sun goes down) - eat your main meal in the middle of the day when your digestion is strongest

* cut out allergens (milk, nuts, wheat) and add them back slowly because you could very well be perpetually functioning beneath your optimal simply because your body is constantly in an allergic response state

* green smoothies every second day (kale, broccoli, spinach, bananas, ginger, coconut oil)

* 20 minutes mindfulness meditation daily

* listen to good music

* smile

:)