Simple Yeast Storage Procedure
- Sanitize or sterilize jars. (Mason jars work nicely for this purpose. A quart size jar which will hold about 300 billion cells)
- Leave behind about an inch of beer when racking the beer off the yeast cake.
- Swirl the fermentor to suspend the yeast. (Avoid shaking as oxygen has a tendency to rouse dormant yeast)
- Pour the slurry into mason jars. (The typical 5 gallon batch produces three to four quart jars full of slurry.)
- Put the lids on just and snug them up. (The lid should not be tight. You'll want the yeast to be able to off gas while in storage.)
- Place the jars in the fridge (or freeze with 10% glycerin) until you are ready to use them.
- Fewer processes steps and less contact with the yeast means there is a lower chance of contamination.
- Alcohol, hops, and the PH of the beer provide antimicrobial properties.
- Alcohol is removed which can cause significant cell death above 8% ABV. 
- Flavor components of the beer are removed. (The effect of the couple cups of beer in the next five gallon batch will often be trivial.)
Unfortunately it does not. Water washing yeast is a common laboratory procedure for separating the dissolved compounds from the suspended material. It does not effectively separate out the suspended material. There will be some stratification, forming layers of partials that sink at different rates. Yeast that has formed flocks will sink slower than single cells. This, however, is independent of viability . In the end, separating yeast using water washing may result in a small amount of yeast that appears beautifully creamy , but the viability is the same as the large amount of yeast that was discarded. Another problem that is introduced with yeast rinsing is that the selected yeast are the slow flocculators. When this yeast is used it could result in cloudy, or slow to clear beer.
How long can you store yeast?
Yeast can be stored in the refrigerator for months. By adding 10% glycerin yeast can be stored in the freezer for years. A week of storage will have an impact on fermentation lag time making it perform about as well as yeast from a vial that you would purchase from a home brew shop.
Do I need a starter?
In a blind taste test it has been shown that there is very little perceived difference in beer quality whether using a starter or pitching directly from refrigerated slurry. In addition to increasing the biomass a starter will also activate the metabolism of the yeast. If the yeast has been in storage for more than a few days there will be a lag time while the yeast acclimates to fermentation. This typically takes less than a day.  There seems to be mostly anecdotal evidence indicating that off flavors are produced during this lag phase.  To avoid this, a starter could be made from the stored slurry.
How much yeast is in these jars?
A safe estimate is normally 1.5 billion cells per ml of thick settled slurry. In the hundreds of cell counts I have done I have seen this range from 0.5 billion per ml up to 4 billion per ml. It depends mostly on how the wort is prepared, but is also affected by the yeast strain.
What is the viability?
Viability will depend mostly on storage temperature and storage time. The half life of refrigerated yeast is approximately 3 months. Frozen yeast will lose approximately 50% of the viability from the freeze, but will drop very little in viability over time. We can conclude from this that if the yeast is to be used within a few of months, it is best to store it in the refrigerator, and for longer storage freezing may be more appropriate.
 Murray, C. R., Barich T., Taylor D., "The Effect of Yeast Storage Conditions on Subsequent Fermentations", MBAA TECHNICAL QUARTERLY, Vol. 21, No. 4, 1984
 Woodland Brewing Research unpublished works
 Taylor, K., "YEAST HANDLING AND TECHNOLOGY", OCBC, 2014
 WELLMAN, A. M., STEWART, G. G., "Storage of Brewing Yeasts by Liquid Nitrogen Refrigeration", APPLIED MICROBIOLOGY, Vol. 26, No. 4, p. 577-583, Oct. 1973
 Woodland Brewing Research Viability storage research.
 N. B. PAMMENT AND R. J. HALL, Absence of External Causes of Lag in Saccharomyces cerevisiae, Journal of General Microbiology ( 1978), 105, 297-304.
Steven Deeds is the Woodland Brewer! For more from Steve be sure to visit him at his blog!