Here is the best way I've found to take a shower efficiently, effectively, and ecologically. Why a shower rather than a bath? Because it uses much less water to get you clean and rinses all the dirt off of you rather than you sitting in it.
1) Put your dirty clothes in a proper and safe location as well as your clean clothes (a place they won't get dirty or wet but separate from each other).
2) Put your shampoo and soap in an easy to reach location in your shower and have a towel within easy reach as well. Also keep a squeegee in your shower (more on this below).
3) Get the water hot without wasting it. Use an on demand hot water heater or on demand water recirculation system (http://www.gothotwater.com/). Or capture the water in a bucket to flush the toilet or water your plants. Or do it military style and just jump in regardless of the water temperature -- let it get warm while you are in it.
4) Make sure all the water stays in your shower. Shut the shower door or tuck the shower curtain inside the tub or shower so you don't get the floor outside the shower wet. This is a safety hazard as well as something extra you'll need to clean up later -- otherwise you'll find mold and fungus growing outside your shower.
5) Wash your hair first (including conditioner if desired) and clean yourself with soap starting from the top and working your way down your body to your feet. I prefer to just use my hands to scrub my body as it serves to inspect my body for possible issues and I don't need to maintain a clean wash cloth, loofah, or scrubby that otherwise harbors mold, mildew, and bacteria.
6) To be super, super efficient consider wetting yourself down first, turning off the water, sudsing yourself up (top to bottom), and then rinsing yourself. I call this the "2 quarter shower" because I can take a shower using just 2 quarters at a pay shower stall like the ones at many state or national parks. One quarter to get wet and a second quarter to rinse off.
7) After you've rinsed, stay in the shower and squeegee yourself off with your hands. This removes most of the water from your body and is a good stretching exercise. Use the front and back of your hands like a squeegee to remove water first from your hair and then wipe water off of your body from top to bottom including your arms and legs. Using this method I can generally finish drying myself with something as small as a wash cloth or small hand towel -- very helpful if you are camping and traveling light :-)
8) Squeegee down your shower next. This helps prevent soap scum buildup on your shower walls and greatly reduces mold and mildew since your shower will then get drier faster after you are done. This will save you lots of effort later in cleaning and maintaining your shower (an ounce of prevention is worth a pound of cure).
9) Dry yourself off while you are in the shower (but if you are going to shave, skip drying off your face). This is why your towel should be within easy reach. The shower floor is relatively clean (you've just washed it down while cleaning yourself) and any drips stay in the shower. When you get down to your feet, dry each off just before stepping out of the shower. The area outside of the shower is often a bit dirty when camping so I will often step into or on top of my shoes or a small clean cloth when stepping out of the shower. Even if stepping out onto a bath mat this keeps the mat quite dry so the next person taking a shower also has a dry mat to step on. Plus dry feet are less prone to pick up dirt then wet feet.
10) Shave over a sink with a mirror after your shower. You can then see what you are doing and can rinse off your razor with short bursts of water from the faucet rather than leaving either the shower or the faucet running.
11) Hang your towel up neatly so it will dry quickly. Don't leave it in a bunch or a pile where mold, mildew and bacteria can have a feast. If you squeegee yourself, your towel will often be relatively dry already. If you do a super efficient shower daily, it works well to get a new clean towel weekly. The less laundry you do, the less water and energy you use.
That's it. Thanks for using our precious resources efficiently and effectively! Enjoy!