We waste more money in the bathroom than in any other room of our house. One of the mantras of energy conservation is doing more with less. The shower is a huge area where some major wasting happens. You can easily fix the problem with a new, greener shower head.
Some modern shower heads come with a simple shut-off valve located near the nozzle that allows you to turn off the flow of water and turn it back on again without affecting the temperature setting. Doing this a few times makes you realize how wasteful a 20-minute shower really is; you can get just as clean and use a lot less water in the shower.
The same is true in the bathtub. Filling a bathtub an inch or two lower is unlikely to make any difference from a hygiene standpoint, and the water and energy saved add up. Also, consider taking a shower instead of a bath in the first place. A shower, if it's kept to a reasonable length, usually requires less water than a bath.