Using "mother cultures" is probably the biggest improvement I've made in my cheese making. However, by "mother culture" I mean making sure that I make up some yogurt the day before and using that yogurt at a rate of 15g per liter of milk. It's just massively more precise and predictable for small cheeses than trying to measure DVI culture. I make the "mother culture" from either DVI cultures (just a tiny amount -- it doesn't matter how much you use as long as you give the milk enough time to grow the culture), or I make it from new commercial yogurt. DVI cultures are dramatically cheaper and more convenient to use, though. You also know what you are getting.
One of the problems with using commercial yogurt is that they don't always tell you what's in it. Also, often with thermophilic cultures, they are bacteria that you don't necessarily want to use in cheese making. Finally, there are some bacteria that you probably want for certain cheeses (helveticus for instance) that you probably won't be able to find in commercial yogurt. Having said that, I use my Bulgarian yogurt all of the time and I love it for cheese. I will probably buy a Bulgarian yogurt DVI culture the next time I buy ingredients though, because it's just *so* much cheaper and convenient.
I've tried to maintain mother cultures several times: freezing yogurt cubes, maintaining yogurt in the fridge and reculturing, maintaining whey cultures... But I've been met with limited success. Freezing cubes of yogurt works, but I've found that some of the cultures are more susceptible to dying off than others. So if you have a mix of cultures, then you get some things apparently dying off. Maintaining cultures as yogurt seems to work as long as you do mesophilic and thermophilic separately, but the mesophilic yogurts tend to have problems at low pH and so LLD, for example, seems to slowly die off. Whey cultures seem to die after only a few days. A whey culture, that you make a mother culture from, and then make a cheese from, and then a whey culture, etc, etc seems to work *really* well, but you pretty much have to make cheese every 3 days :-). So I've kind of resigned myself to using DVI cultures. Since I work from home, I am thinking of trying to find a rhythm where I *could* make cheese every 3 days, though... Not sure I can eat that much cheese...
Edit: For recommendations to buy - Flora Danica, and the Thermo B Starter Culture from Biena. Be a bit careful if look at cheesemaking.com information, though, as the dosage amounts seem wrong for all of the Biena products. 1 Do for Biena is for 100 liters of milk. But again, I highly recommend making a mother culture from it and using the mother culture for making the cheese -- dramatically better and easier.