After some weeks, your hair gets used to having no shampoo.
I'm doing it since 2 months and all is fine.
Buy a tub of sodium bicarb and put it in an old shampoo bottle. Add water and mix - aiming for shampoo like viscosity.
Depends how much hair you have, your activity levels and where you live but start with using it once every couple days. You may end up at once / week, but depends on the above factors.
Use it like shampoo, but more vigorously.
The equivalent would be not using dishwashing liquid for your plates after a greasy meal. Hot water and friction can do a lot, but it is an uphill battle. In the case of hair, your method might just be "good enough", though. But I'm sure your hair won't get the "Gee, Your Hair Smells Terrific" response.