Ritual. Some of this helps with sleep, too, if you do it before bed.

Choose a smell, music, a special way of sitting. A cup of coffee, a snack. A combination of things. Perhaps use headphones. The more often you do this before time you have flow, the more likely it is that your body and mind will view that ritual as a signal to get into a flow state.

Resource: Occultism. Some mind states used with occultism are likened to the state of flow and folks work some time getting to those states faster.