I used to use saved yogurt from previous batches, mixed with a little whey as my starter culture. Week after week, my yogurt got weaker and weaker, and eventually I would have to get some store-bought yogurt and get things going again. I was going along like this when I decided to try a yogurt starter.
The first batch of yogurt I made with the starter was one of the best batches of yogurt I have ever made. I followed the jar directions, adapted to my home technique: I use a ceramic casserole and then put it on a Yogurt-maker base, covered with a blanket. I let this incubate for about 5 hours.
After it has incubated, I let it cool to room temperature and refrigerate it. Once it is cold, I strain it through a piece of cotton sheeting held by an old colander over a bowl. The result is a lovely, creamy thick yogurt that has the richness of sour cream.
I only use reserved yogurt one time, so every other week, I am using the Probiotic Starter. I have had much more consistent results with this starter.