Have you ever thought the shower warm up water draining down the plug hole seems like such a big waste ?
If you place a bucket under the shower head, this clean water could be used elsewhere. Of course you could pour it into the washing machine and this is something I do often. It's not going to save you much money but you'll feel better about reducing the waste.
Here's an even better tip that will feed you and save lots of money. Reuse the shower warm up water to water some veggie seedlings. If you love lettuce, you can grow it all year round in many climates. A $3 packet of seeds will last a few years and could save you over $100 a year. This figure is based on buying one small $2 bag of lettuce leaves every week for one year. If you have a larger family, you would be buying more lettuce so the savings will be even bigger.