When I save yeast I do it at the start. I make up a two liter starter and let if ferment out. I drain off most of the liquid and the pour some into small sanitized canning jars. I add some more fresh wort to the jars and loosely attach the lid and ring. I let them ferment out, tighten the ring put them in the fridge. When I am ready to use them I just make a starter like normal. I have had good results using these after 4 months in the fridge, or I can just add more wort to them and keep them longer.