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Earlier, [another user] mentioned that he read that citric acid is sometimes used as a water softening agent, and wondered if it could be used to help those with hard water make lather more easily. I remembered that we have a bottle of citric acid in the pantry (use it for canning tomatoes in the summer), and of course I have moderately hard Lake Michigan water (~8 grains/gallon) coming out of my taps. So I did a little experiment.
First, I made a lather with plain hot tap water. I loaded a damp Ecotools Kabuki brush for 15 seconds (timed) on a puck of Ginger's Garden Lime Yuzu glycerin shaving soap and built lather in a bowl, adding drips of water until I got to "soft peaks." 15 seconds is about half the time I'd normally load, and as I expected, I ended up with pretty meager lather.
Then I rinsed and dried the brush and bowl measured out a liter of hot tap water, added a gram of citric acid to it, and stirred. I used that water to make a new lather, again loading 15 seconds on the same soap, and it was like using distilled water. Immediately a much denser proto-lather, and it was able to absorb a lot more water before the peaks began to soften at all. The lather had a much larger sweet spot, in general seemed denser and more stable, and there was just a lot more of it.
So why do this when distilled water is pretty cheap? You can get 8 oz. of citric acid for $3, which should be enough to treat 50+ gallons of water. And it's a lot more convenient. No need to microwave distilled water or use a hot pot; just fill the sink basin with hot tap water, drop in a pinch of citric acid, and then proceed as you normally would. And it's totally safe; you probably eat tons of it every day, as it's used as a preservative and for sour flavor in lots of foods.
If you have hard water, give it a try. Those with really hard well water may need to use a little more, but the exact amount shouldn't be too critical. Measure it out once to see how much a gram of it is (spoiler: a very, very small amount. Less than a quarter teaspoon, I'd say) and you can probably eyeball it from there. I'm planning to use it for all my shaves for a couple weeks and see how it goes.
The other interesting question is, could soap-makers add citric acid to their soaps to make them perform better in hard water? I asked one soap-maker and he actually had already heard of citric acid's water softening properties and done some research on it. Unfortunately, it interferes with the soap-making process.
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epic fail 
