I have a very long hair. I like it in a bun, so it's easy to not notice that it's gotten too long. I was looking for a way to wash it efficiently, without using too much shampoo, water, etc. Here's my formula. It works great. I hope it helps you to teach your long-haired daughter not to use a whole bottle of shampoo every time she washes her hair; or helps to make your travel lighter and easier.
Step 1 - always wash your hair AFTER you wash yourself. (this way the hair will get wet thoroughly, and the clean hair will not get dirty again from soap splashes and high humidity.)
Step 2 - pretreat - take a small amount of shampoo and rub it into the scalp. Don't bother making sure that you get it on ALL the length of the hair, it's not nessesary. This is what you usually waste your
shampoo on. The oil on the hair does not let the shampoo to foam up well, and you have to use more and more. Don't bother. Rinse. Move on.
Step 3 - wash. Notice how much less shampoo you are using. Rinse. No need to repeat! Use conditioner if you like.
Step 4 - adjust the shower stream to the max (if possible) and lower the temperature a bit. Using a thick-tooth comb, comb your hair. It's easier to comb in the shower and will minimize tangles.
Step out of the shower. Dry your hair as usual.