actually, I've had some really bad luck doing this and think I now understand why. When you remove yeast from the cake, you're bringing with it more than just yeast, maltose, and water - you're bringing starches and I guess, other things, that bacteria like. Everytime I've tried to 'harvest' yeast I've ended up with some off-flavor nasty yeast the next time around. So, on the advice of Asheville homebrewing supply in NC, i've started making large starters on just LME, pitching part of it, and saving the other part. Then I use this starter continuously, simply building it up and pitching a part, in consecutive brews. That way the original starter never sees anything but maltose, water, and yeast and the exposure time is minimum.
I have also had really good luck pitching right on top of the cake and I think the difference between doing this and harvesting/reusing is exposure to air/bacteria. Seems like it wouldn't be much but you should've smelled/tasted all the harvest yeast I just poured out. Thankfully I only used it twice and only had to get 10 gallons of off-flavor yeast beer down my gullet.
cb