One thing I will add for those looking for a way to maintain yeast cultures without resorting to plating and slanting is that one can use 1100ml of 10% w/v hopped wort (110g of DME in 1100ml of water with a few magnum or galena pellets) for one's starter. When pitching this starter at high krausen, one should hold back 100ml in a santized container that allows for off-gasing of CO2. This saved part of culture should be immediately placed in a clean refrigerator. In order to keep things as aseptic as possible, the transfer of the 100ml of held back starter needs to occur first and the pouring surface of the starter vessel should be wiped with 91% isopropyl alcohol (my state has outlawed the sale of 190 proof/95% Everclear grain alcohol or I would use that) before flaming with a BIC lighter. The goal is to get the pouring surface to be as close to being as absolutely sterile as possible. One hundred milliliters of starter at high krausen should contain approximately 20 billion cells. That is less than a White Labs package, but it will be enough to get the job done as long as one pays attention to detail. I used to pitch 40ml of 1.020 starter media that was inoculated with a couple of 4mm loops of yeast into a 1L starter, so I know that this method should result in a culture that can be used to start a 1L start for up to around 6 months. Because our seed culture is cropped from a 10% w/v solution (~ 1.040 S.G.), it should not be stressed like yeast can be after a fermentation. One may be able to push the re-propagation date out to a year with really hardy strain. However, I would do a two-step starter starting with 250ml of 5% w/v wort (12.5 grams of DME into 250ml of water) that is then pitched into 1L of 10% w/v wort at high krausen. If one does not have a scale that can measure in tenths of a gram, now is the time to acquire one.
I gained this insight from propagating my sourdough culture. A lot of books have people feeding the culture on a regular basis, which results in a lot of discard. My girlfriend put our discard to good use making sourdough pancakes and waffles, but these days I actually have to make discard because it is not part of my sourdough culture maintenance process. What I am doing is keeping 100g of sourdough culture every time I make bread or pizza dough. I keep this 100g in the refrigerator and use it to propagate starter for use in making bread or pizza dough. The 100g of culture comes out of the refrigerator. Fifty grams of whole wheat flour and fifty grams of unbleached white flour are added to it before adding 100g of chlorine-free water. When I first started this propagation process, it would take a long time for the culture to double in size. However, selective pressure has resulted in a culture that more than doubles in size in just a few hours. I suspect that placing this kind of selective pressure on a brewing yeast culture will eventually lead to a culture that is very hardy in one's brewery.