Use It Till It’s Gone!
Really sounds simple, finish the bottle before you start the new one. Shampoo, toothpaste, body or hand wash, ketchup, salad dressing, it matters not, if it is a somewhat thick sticky goo we all get anxious about breaking into the new bottle…fight the urge and finish the old one first. Many bottles with pumps only reach close to the bottom…add a little water/milk/vinegar and bam you have a week or two more of product left to use. Open the new bottle and marry it to the old bottle. If that thought worries you because it is food and you don’t wish to cross contaminate then sit your bottle upside down or empty the contents into a small dish.
And for heavens sake rinse out the bottle and reuse or recycle it when you are done.