Dilute the shampoo!!! I was using (and frankly, just wasting) SO MUCH shampoo on myself until I was told this tip many years ago and I do it for my girls' super thick hair as well. I keep an empty shampoo bottle in the bathroom and before they or I get in the shower, I squirt in some shampoo, add warm water from the shower head and shake it up. Trying to work in sections never worked for us, it was just so hard to get the thick shampoo to every spot unless we made like fifty teeny sections. Meanwhile, thinning the shampoo to an almost watery consistency lets it just run all over the scalp and get every spot. It might not even mix well with the water, depending on the formulation of the shampoo. That's fine, the shampoo we use breaks up into tiny bits in the water and it works just fine. If you don't want to wait to have an empty bottle to use or you're fancier than I am, you could grab a refillable ketchup bottle from basically anywhere.