What are the most expensive ingredients in making beer? Hops and yeast. A simple dry packet of yeast is only $3, which isn’t so bad (for example, the classic Safale-05), but a special vial of yeast is around $6 (for example, Belgian Trappist). Growing hops and reusing yeast mitigate those costs tremendously. I especially like to harvest and reuse those special yeasts (Trappist, Scottish, San Diego Super), which I keep in my fridge for several months and reuse them several times. “Harvest” is just a fancy word for getting yeast from your fermenter into a jar.
Harvesting yeast, like most things in this beer-making hobby, can be as complex or simple as you want. As you probably guessed, I do it the simple way.
So, when the next brewday comes along, I simply take the jar of yeast out, shake it up, and pitch as much as I want (I eye-ball it…the worst that could happen is over-pitch, who cares).