If you want to make your bottle of shampoo last a little longer while still improving your hair's health, all you need is a pack of gelatin.
One Good Thing By Jillee recommends mixing in a half cup of water and half a packet of unflavoured gelatine for every 230mL of shampoo left in the bottle. It will need to be well blended, so you'll probably want to squeeze all of the ingredients into a mixing bowl, and then add it back to the bottle with a funnel later.
Once you're done, the gelatine will increase the protein content of the shampoo and thicken it, allowing you to use less in each shower. In addition to saving you money, the extra protein helps promote a strong, healthy head of hair. This definitely sounds a little gross at first blush, but it's worth a try if you don't want to pay extra at the store for protein strengthening shampoo.
If you have any gelatine left over, Jillee also created some DIY hair gel with the same ingredients, so be sure to check out the source link for a walkthrough.
How To Stretch Your Shampoo & Make Your Own Hair Gel [One Good Thing by Jillee]