There are two types of technology change: Incremental and Disruptive. As a designer, disruptive is way more fun.
Incremental: How do we get more with less? For showers, building codes limit their consumption to 2.5 gallons per minute (gpm) while green building rating systems are pushing manufacturers to get more with less; down to 1.5 gpm. The problem is that at a certain point there is a physical limit to the incremental change and efficiency gains will level off. My gut says that for shower heads this point is somewhere around 1.2 gpm. Any 50% reduction is something to be proud of, but it has taken a long time.
Disruptive: What if we ask a different question altogether: “Is there a better way to take a shower?” A couple startups are trying to answer this question with what they call a cloud shower. Previously only available as a luxury shower add-on, the technology is similar to a type of commercial humidifier. They claim that their technology not only uses 75% less water, but also provides a much more luxurious showering experience: providing more even coverage and rinsing more effectively. We now have a sudden 75% drop in water consumption, which also results in a major drop in the amount of water that needs to be heated, which in turn saves a lot of money. All this by simply swapping out a shower head! Brilliant!