I almost exclusively use liquid yeast, WYeast, ECY and occasionally WLP, because of the large variety and wonderful strains available. From a new pack/bottle I first make a large (1.3-1.5 liter) starter, then split it in 4ths after cold crashing and decanting. Three go into 4oz mason jars, around 90 billion cells each, and the 4th becomes a new starter. I usually pitch 3/4 and save 1/4 into another 4oz jar. I mark the generation of each saved out starter: "St.#" and the date of harvesting.
I intend to stop harvesting yeast, and make fresh (overbuilt) starters each time, unless the yeast is very clean and I can reuse it within a few weeks.
The main reason for me to stop reusing yeast is I noticed after 2-3 re-uses there was a strong reduction in flocculation and occasionally some off flavors. Some of my beers wouldn't clear anymore, even after 2 weeks cold crashing at 32F.