In addition, if you have a quality brewpub or microbrewery nearby, the brewers are often happy to provide yeast to homebrewers. A good brewery produces a lot more yeast than they can use and it is usually free of contamination. I keep a spare sanitized pint plastic container in the car in case I am visiting a micro and am able to talk to the brewers. (I know what you are thinking, "What are the odds that I will be at a brewpub when they are brewing?" Sometimes it requires several visits a day to even those odds, but that's life.) If they don't have any yeast available at the moment, they will usually suggest you come back the next day/week when they are transferring, and will give you some then.
The advantage to obtaining yeast this way is that you usually get a cup or more of slurry which is more than enough to ferment a 5 gallon batch. You are virtually assured of a vigorous, healthy fermentation, without the fuss of preparing a yeast starter a few days beforehand. The yeast will stay viable for a couple weeks if kept in the refrigerator. But remember, you may want to replenish the yeast's glycogen and trehalose reserves, as described in section 6.6, if the yeast has been stored for a long time.
Simple Yeast Ranching
Each batch of beer you brew is a good source of yeast for a future batch. The best way to obtain yeast is to skim it from the krausen of a currently fermenting beer. To do this, you will need to be using a bucket type fermentor and first skim off the green/brown hop and protein compounds with a sanitized spoon early in the primary phase. As the creamy white krausen builds up, you can skim this fresh yeast off with a sanitized spoon and transfer it to a sanitized jar. Fill the jar with cooled boiled water and place it in the refridgerator. The lack of nutrients in the water will cause the yeast to kind of "hibernate" and it will keep for up to a couple months. You should pitch this yeast to a starter after storage to re-vitalize it.
The only drawback to the above harvesting method is the contamination risk for the current batch. Experienced brewers with good sanitation practices can harvest yeast that way without much risk, but for newer brewers it is probably better to collect the yeast after the fermentation is complete. You can collect yeast from either the bottom of the primary or secondary fermentor. If you obtain yeast from the secondary, it will have only small amounts of trub mixed in and will be easy to seperate. However, you need ot be aware that if you repitch yeast harvested from the secondary several times in succession, you will tend to select the less flocculant cells of the population, and future beers will be slow to clarify. But, if you only repitch once or twice, it is not a big deal. I myself usually harvest yeast from the secondary.
If you harvest yeast from the primary fermentor, you will need to separate the yeast from all the trub that is mixed in. Professional brewers most often do this by "acid washing" the yeast--using acid to lower the pH to about 2.5 so that bacteria is inhibited and using whirlpool methods to seperate the heavier trub from the lighter yeast. But acid washing tends to inhibit the yeast too, and is not strictly necessary. You can simply use chilled boiled* water and two sanitized jars to separate the healthy yeast (white) away from the majority of the trub.
After racking the beer, swirl up the yeast layer on the bottom and pour some into a large sanitized jar (such as a mayonnaise jar).
Gently pour in some cold, boiled water and swirl it up to get all the yeast and trub in suspension.
Let the jar sit for a minute or three to allow most of the trub to settle to the bottom. Gently pour the cloudy water, containing suspended yeast, into another sanitized jar. Discard the dark trub.
Add some more water and repeat this procedure until you are left with a substantially light-colored yeast suspension and only a thin brown layer of dead yeast and trub on the bottom of the jar.
Store the jar in the refridgerator for up to a couple months. The yeast will turn brown as it ages. Discard it once it turns the color of peanut butter. Eventually the yeast will autolyze and die as its nutritional reserves are used up.
Pitch the yeast to a starter before using to ensure its vitality. If the starter smells wrong--rancid, vinegary, etc., the yeast may be contaminated. The dominant smell of a starter should be a yeasty smell, but sulfur smells are not necessarily bad, especially with lager yeast strains.
*Note: You want to use boiled water for two reasons:
To avoid exposing the yeast to dissolved oxygen which would cause the yeast to deplete their glycogen reserves before storage.