My new job has me buying my own coffee for the first time in my professional career. I'm amazed at the amount of coffee I consume. I brew at my desk using an aeropress. Has anyone tried to use the grounds more than once? Any tips as I try it out?
In general, I argue that you shouldn't be able to re-use the grounds. That is, for any brewing session, your goal is to extract exactly what you want from the beans. If you do this optimally for your method and taste, there's nothing left in the beans that you want; re-extracting will give you a different result. The second brew might be drinkable and even tasty, but it will have a disproportionately small amount of stuff you might want, like caffeine and other flavour compounds.
That said, @Madmanta's answer makes a good point: it might work in some cases, such as Aeropress or espresso. But this suggests you might want to change other parameters of your brewing. If you can re-brew your coffee with a result that you like, you might be able to, for example, use less grounds. This saves money, which sounds like it's part of your goal. Alternatively, you could brew a larger cup of coffee with the same amount of beans, or increase the extraction time. In the case of AeroPress, for example, I find that many of the recipes are decidedly under-extracted, so this might be an explanation for why a second brew works for you; see this Q/A for what I mean by extraction in this case.
You could try this experimentally also: brew a cup of Aeropress or pour-over as you usually do, then brew another batch in the same way. For example, I find the last few drops of pour-over brew to be watery and off-tasting but the entire batch tastes better together than either of the two parts separately. If you like it, go for it. Or use half the amount of grounds the next time. :)
As a practical note, you might also consider simply to brew a larger batch of coffee all in one go. You'll get a slightly different product but, for example, I find that I use way less grounds for a 10-cup thermal carafe than for 10 individual cups.
With a manual espresso machine, I have used the same grounds for two pulls. They are ok in a pinch if I want that volume but don't want to redo the grind, tamp etc. But honestly it is not as good as the first time through. The only thing I can suggest is that when I do them one after the other, it is ok. If I wait for the grounds to cool it is not really drinkable.
I used to do this for a co-worker and me. We'd use three AeroPress spoonfuls of coffee (which is a lot) to make two cups. I think the theory was that the saturation of that much coffee grounds in the small amount of water that took up what little room was left in the AeroPress chamber, was not efficient at all. So we'd brew the same grounds twice. This made for more bitter coffee, because the coffee was over extracted, and it pulled the tannins into solution.
We opted for simply letting the coffee brew longer and increasing pressure during the press to force extract more out of the grounds. This yielded a much stronger concentrate that we'd then dilute to normal coffee/water proportions. This created a delicious and full cup of coffee every time.
I think for your case, you'd be brewing hours apart with the same grounds to save money, and I think you totally could. The issue I see is that most caffeine in coffee is extracted within seconds of brew time, so your second brew with those grounds may not yield a nicely drugged cup of joe. So not only will the taste be bitter, but it may also put you to sleep.
I think this question has been satisfactorily answered for hot brewing, like in an Aeropress. I think it's worth adding that double-brewing is worth trying if you cold brew.
I have recently tried double-brewing with my Filtron cold brewer, and found the results to be quite good. A single pound of coffee yields 2 liters of extract with this method, which I dilute to roughly 2.5 gallons of coffee (320 ounces, in other words 20x16oz cups).
Cold brewing doesn't abide by the same rules as hot brewing because it happens at a low temperature and doesn't extract any of the bitterness typical of over-brewing at a high temp.
I've tried using coffee grounds twice and obviously there is a quality and potency difference, if you just really want to stretch your coffee more than you can use less coffee per cup or if you are like me and just like the feeling of having a warm cup then reuse the coffee grounds twice but the second time around it better to brew it twice as long.
This morning I used my coffee grounds twice. I forgot to buy more coffee, so I placed my French press with the used grounds in the refrigerator over night. In the morning I brought the French press out with the used grounds. I had about one tablespoon of fresh coffee grounds left in my bag of coffee. I added that to the used grounds. It came out pretty good. No doubt the small amount of fresh grounds helped.
In a pinch, it beats going without coffee in the morning.
I do this with a French press, usually with a medium roast. My best results have come from exaggerating the quantity of beans the first time (many times at an increased extraction temperature), then extracting a second time with a larger water:coffee ratio. I haven't taken measurements, but it don't feel a caffeine rush from the second extraction.
A few tricks not already mentioned:
Instead of brewing the grounds a second time, extract more in the first brew and then dilute it, but control what you extract in the first brew. If you brew with water between about 175F and 185F, it does a good job of extracting the good flavors and reduces the amount of bad flavors that go into solution. Brew longer in that temperature range to get a more thorough extraction that is still drinkable.
Jason Abalos's answer describes having tried using more grinds, then diluting, and then settling on brewing longer, pressing harder, and diluting the result with water. The grounds in the AeroPress can trap a substantial amount of the brew, especially if you start with a higher ratio of coffee to water.
Rather than diluting the brew you get out, put that extra water back into the AeroPress and immediately press it through the grounds, and use that to dilute the previous extraction. That will flush out more of the trapped brew with minimal additional extraction.