I'll personally only use the cake twice (for a total of 3 batches) - though I've heard you can do it more. Probably not true, but it just seems like the risk of infection and/or risk of off tastes from the trub that's accumulated would go up with each re-use.
And, when washing yeast, I'll only wash the yeast when the cake hasn't been re-used - I'm sure it doesn't matter, but it's what I've been doing and it works.
Atomic Dog Brewery
On Deck: Centennial IPA, Dry Irish Stout
P: Cascade Pale Ale
T: Carbed Crystal Light, Root Beer, Vanilla Cream Soda, Tripel IPA, Apfelwein
e^(pi*i) + 1 = 0