I've always known that re-pitching yeast from prior batches can produce truly great beers, but I never bothered with it. Now I'm bothering, and here's what I'll do.
1. After fermentation and when the beer is racked off (to whatever vessel), swirl the yeast cake at the bottom of fermenter and pour into a graduated sterile container and set in cold fridge for next brew day. I'll use a $5 polypropylene bottle and sterilize by putting a little bit of water in the bottom, lightly capping, and microwaving for a couple minutes while the water inside boils and steam heats the container.
2. Figure out how much yeast I'll need for my next batch of beer (should brew within 2 weeks of your last batch, as yeast viability greatly decreases after this time without making a starter) by going to Jamil Zainasheff's Yeast Pitching Rate Calculator at www.mrmalty.com. Use the "Repitching from Slurry" tab, and enter the fermentation type, original gravity and volume of your next brew, and enter when you harvested your yeast from Step 1 above. Read the "# of ml of yeast needed" value on Jamil's calculator. Don't touch the "yeast concentration" and "non-yeast percentage" default values.
3. When its time to pitch for a new batch of beer, take the bottle out of the fridge, open it up and decant the beer that sits on top of the yeast at the bottom. Read the volume of what remains in the bottle after decanting. Divide the value that Jamil says I need (from Step 2 above) by this value to get a percentage.
4. Pour some sterile brewing water (I always have this handy, sitting in a growler in the fridge) into the bottle on top of the yeast, cap and shake vigorously to clean the yeast. Let settle for about 10 minutes, and the trub and dead yeast cells will fall to the bottom. The healthy yeast remains in suspension in the liquid layer on top. Read the new volume, and multiply this by the percentage calculated in Step 3 above. This is the volume I need to remove from the container and pitch to the new batch of beer.