When I first started, I just bought a container of plain yogurt at the store and saved a little to use as my starter. Then I saved several tablespoons of my homemade yogurt to use as a starter for the next batch. However, after about 3-4 batches, the saved starter wouldn't work as well. Then I'd have to go back to the store and buy more yogurt.
Next, I bought some Yogourmet freeze-dried yogurt starter packets. The upside is that they make tasty yogurt, and it only takes 4-5 hours (versus 8-10 hours for store-bought yogurt as the starter). The downside is that they were expensive--about five dollars for six packets. I didn't want to have to keep buying expensive starter, that defeated the whole point of making my own yogurt. So, I devised this plan to make dozens of batches from that one box.
First, I made a batch of yogurt with a Yogourmet packet, following the packet directions. When it was finished, I saved some in the fridge to use as the starter for the next week. I also filled an ice cube tray with yogurt. Each cube held about 2 tablespoons of yogurt. I froze the tray, then popped the cubes out and stored them in a Ziploc.
For the next several weeks, for my starter I used the yogurt I'd saved in the fridge from the previous batch. But, after several batches my saved starter didn't work as effectively. Then I pulled out several frozen cubes. I put them in a little glass bowl and let them thaw at room temperature.
Do not microwave the cubes, you will kill the bacteria and your yogurt won't culture! Also, yogurt that has been frozen and thawed looks really weird and watery for some reason. Don't panic, just use it like regular starter.After the yogurt was ready, I saved some in the fridge to use as a starter for the next week. For the next several weeks, for my starter I used the yogurt I'd saved in the fridge from the previous batch. When it started to lose effectiveness, I pulled out more cubes from the freezer.
Having frozen yogurt cubes was also convenient when we went out of town on vacation this summer. Before we left, we just ate up all our yogurt. When we came home, I just pulled several starter cubes from the freezer to make yogurt.
As you can see, this method will allow me to make several months of yogurt from just one freeze-dried packet. So far, I've had the starter cubes in the freezer for about 2 months and they still work great. The freeze-dried packets should last over a year since I store them in the fridge. I'm hoping to make about a year's worth of yogurt from that one $5 box! Sometimes being frugal is so fun :)