This NASA article states that the ISS crew members will use 4 liters of water or less for bathing, in comparison to 50 liters which they claim is the average used by earth's citizens in developed countries.
Is there a practical example of how you can bathe yourself while using the least amount of water efficiently?
Step 1: shower rather than filling a hot bath. If you happen to shower in a bathtub, leave the plug in next time and compare the final water level to how deep it is when you run a bath. For most people showers use 1/4 to 1/2 the water of baths.
Step 2: shower less often. Your hair adjusts its oil production to your washing frequency, a fact you can easily confirm in a week's experiment. Every other day should be fine unless you are a manual labourer or construction worker. You might benefit from wearing deodorant on non-shower days. Showering every other day will cut your water use in half. If you're convinced you reek, a quick wipe with a warm facecloth for the parts of you that you believe are the problem will use almost no water, and it's ok that your elbows are going unrinsed for a day.
Step 3: turn the shower off while you do anything that takes time (shaving, applying hair conditioner, etc) in the shower. This has the smallest impact but could cut water use in half depending on the things you do in the shower. Also consider doing less in the shower: do you need to soap and then rinse every square inch of skin from head to toe? How did your elbows or the tops of your feet get so filthy they need soap? Surely they get clean enough just with the water running over them?
Step 4: cut your hair super short (but don't shave your head) so you don't need to do so much to it in the shower. Grow a beard so you don't need water to shave your face. Stop shaving or removing any body hair. Most people would reject these choices because they affect life outside the shower too.
Step 5: wash your hair in the sink (using about 2L of water) every other day, and clean yourself with a washcloth and warm water at the same time. Takes a little longer but could get water use down dramatically. Think of showers and baths as recreational activities rather than for cleaning yourself, and "spend" water when you want that experience. Probably what I would choose before Step 4.
Step 6: install some sort of greywater system so the water you shower and wash with is used for another purpose before you dispose of it. Ironically the more you concentrate the soap (and possibly dirt) in a smaller and smaller amount of water the less desirable your greywater is. But look into it anyway.
The simple answer is a sponge bath, much like the astronauts do. I do this when I'm camping and need to bathe in icy cold water. Grab a handkerchief or small towel and wet it, then wipe yourself down with it. If you're using soap, I find s small bowl with 2-3 cups of water and a bar of soap makes a useful addition. Foam up the soap on your (wet) hands to make soapy water, then put the soap back in the plastic bag it lives in. Dip cloth in water, wipe yourself with soapy water. Then discard the water and add a cup of clean water. Wipe yourself off, and when the "clean" water is too dirty, change it. You should be able to bathe using less than four litres of water this way.
That's time-consuming and I don't necessarily feel very clean afterwards, just less dirty. To save water when showering I turn the shower on, then without waiting for the hot water to come through I jump under the water, rapidly wipe water onto myself, then turn the water off. The water being cold helps me focus on being quick :)
Now I'm wet I can soap up. Then when I turn the shower water back on the warm water will come through more quickly and I can wash off in 30 seconds or less, with warm water arriving at the end. Assuming a low-flow shower head and not washing my hair (I have a buzz cut) that's 2-5 litres of water used.
To give you some idea, during a drought we had a share house with 6 people using less than 155 litres a day between us. That's average consumption over several months, not the one day we all made an effort, BTW.
When I want to be really efficient, my strategy for showering is the following:
I just took a shower (including washing my hair!) to get these times, without particularly hurrying. I measured that the shower strength I adjusted corresponds to 500 ml per 20 seconds. That is, I just took a shower using only 1.5 litres of water.
(When I leave the water running, I use about 3-4 litres.)
When you have to "wait" for warm water, use this water to wash your feet first. You'll feel warmer moving and your hands and feet won't really mind the cold water.
If you sell my strategy to NASA, please consider donating 80% of your gains to a sustainability project of your choice.
P.S. If you're serious about showering efficiency in general, wipe of excess water on your body with your hands before you dry yourself with your towel (only takes 5 seconds). This helps the towel dry faster, last longer, need less frequent washings and allows your bathroom to dry faster, etc.
I wanted to add that in addition to these methods, that when you're done with the water, give it to your plants. They really don't care if there's a little extra dirt and oil in it, and normal shampoo and soap don't affect the plants either.
Your agriculture (household or industrial, it doesn't matter) uses many times more water than the entire rest of your house, even at our most wasteful. So your efforts would be more useful if you didn't have to keep your grass green, like if you had a rock garden or other ground cover that wasn't turf.
