If you are careful you can maintain the starter culture for a long time. Using glass jars makes it relatively easy. You'll need to have 2 jars (at least). Boil the jars (with lids) along with a metal table spoon and let them cool. For a thermophilic culture (high temp), wait until the milk cools down to 50 C (122 F). Open a new container of yogurt and use your table spoon to mix a spoon of yogurt into the milk. Pour the milk into the jars. Seal the jars. Always make sure to have one untouched jar available for seeding the next generation. Always seed with yogurt from a just opened jar -- don't let the jar sit around open.

After the milk in in the jars, stick them in a picnic cooler. Pour in a water bath of 50 C (122 F)water about half way up the sides of the jars. Close the lid and wait 8 hours or so. If you want, you can replace the water after 4 hours, but after fiddling with things I've found that I don't really need to do this. YMMV.

I've kept yogurt cultures this way for years and years with no noticeable drifting. Yogurt sealed this way will keep in the fridge for a surprisingly long time too. While I don't recommend it, I've tasted it after storing in the fridge for more than a year and it was fine. You can do something similar with mesophilic (room temperature) cultures, but you have to be more careful because there are more bacteria around that are happy at 25 C than 50 C.