@Seek makes a good point: trying to make it happen doesn’t work. The trying itself gets in the way. Being “in the zone” is an experience of effortlessness.
It’s also called a “flow” experience, because it’s a lot like just relaxing into the current of a river and letting it carry you along. There are a couple of ways that this experience can get blocked or disrupted: one is that your attention grabs onto a passing thought. This is like grabbing onto something on the riverbank; you’re pulled out of the flow for as long as you hold onto it.
Another disrupter is trying to exercise control. In the zone, you’re not really controlling anything; things just get done as needed. If you’re in there trying to tweak and adjust, things go wrong. Taoists call this “pushing the river”. The river flows just fine without your help.
@zenvelo mentioned paying attention to the breath as a meditative exercise. If you do this for awhile, you can see how this all works. First, you see how grabbing onto thoughts (instead of letting them just drift by) pulls you out of the flow. And you also see that the breath flows best when you just let it happen, without trying to control it. It’s the same way with driving a truck.