A question came up on my beer club forum titled "Saving Yeast". Here is my response:
I do this all the time. I buy yeast maybe once or twice a year. Of course, sanitation is critical. Here's how I do it:
1. Rack off the beer to wherever you rack it to.
2. Depending on how much beer is left in the fermenter, I'll add up to a pint of water. I use store-bought bottled water.
3. Swirl around until the yeast cake is all liquid, nothing clinging to the sides or bottom of the fermenter.
4. Pour out into sanitized 1qt mason jars. I generally get 2 or 3 jars per 10 gallon batch. I leave about 1 inch head space.
5. Drop in one of these per jar: http://www.rei.com/product/736898 This is a tip that Jeremy posted a while back. This helps prevent infection.
6. Refrigerate until you need it. My refrigerator is set for 37F. On brew day, I get out a jar or two (I use 1 jar per 5 gallons) in the morning and put them on the kitchen counter to bring them up to room temp. Shake every now and then to get the yeast in suspension.
7. Pitch when you're ready.
You can do even more if you want. Between #4 and #5, set the jars on the counter for 15 or 20 minutes. It should settle out into 3 layers, the top layer is beer/water, the middle layer is your yeast, the bottom layer is trub. You can pour off the beer/water and yeast into another jar and leave most of the trub behind. You can repeat this several times until what you have is pretty much just yeast.
I've used yeast as old as 2 months doing this and have not had any problems -- well, not any problems that I can attribute to the yeast, anyway.
Some pictures while harvesting yeast:
Mason jars are sanitized and draining.
I added a bottle of water because it looked like I got most of the beer siphoned off and it looks there is some yeast stuck to the bottom.
Yeast in the jars. I let them sit for a couple of hours so I could take some pics of the yeast separating out, but they really looked just the same, so I just put them in the fridge. It seems to me that the combination of whole hops, which form a filter bed in the boil kettle and the counter-flow chiller, there really isn't much trub that makes it into the fermenter.