+1 on all the info Larry provided.
I wash yeast and repitch a lot, however only for the next batch. Storage is a bit trickier. But if you used boiled and cooled water that is oxygen depleted, and kept cold (near freezing but not frozen) the yeast will lay down and go to sleep. A good wash will provide enough yeast for about four normal 5 gal batches, but I found that all I need is enough for the next batch. So when I wash I collect just enough for one large pitch.
Also, keep in mind, that if the beer your thinking of harvesting from tastes like ASS, just toss it and start over with a fresh vial and starter. This technique works best if the beer tastes good to begin with. No sense carrying over flaws from previous beers.
Dumping fresh wort on an old yeast cake will also work, but keep in mind, there may be some carryover of flavors. Also there WILL be a carryover of flaws. Also this needs to be done the same day that the beer is racked off. I have done this, but I start with a low gravity/mildly bitter/light colored beer, and progress up. For instance with WLP001 California Ale or Safale US05 yeast, I will brew an American Amber, to an American Brown, to a Robust Porter. With WLP833 German Bock Lager yeast, I brew a Maibock, to a Traditional Bock, to a Doppelbock. After three batches I loose my nerve and toss the yeast. No need to tempt fate. And the "tastes like ASS" rule from above is applicable.
This all requires you to plan ahead, and to be prepared for future brews. My brew schedule is planned about five brews ahead. I find this enjoyable, for when I'm not brewing, I'm thinking about brewing. And when I'm not thinking about brewing, I'm asleep and probably dreaming about brewing.
