Originally Posted by TyTanium
Another option is top-cropping.
Basically you skim the foam off the surface of fermenting beer (2-3 days into fermentation) and store it in a sanitized jar in the fridge. Very simple.
You get the most bang-for-your-buck by repitching it immediately, rather than storing it in the fridge. When you top-crop, you're capturing extremely healthy yeast at the height of their activity. If you're just going to seal them in a jar and put them to sleep, you might as well just wait and wash the yeast cake instead.
Originally Posted by brewmex41
I feel like that would be easier with a fermenting bucket rather than a carboy though.
Potentially, but you'd risk oxidizing your beer. There are good descriptions in books (like White/Zainasheff's "Yeast"), but basiclaly, to collect yeast from the top of an actively fermenting ale in a carboy, you simply insert a (sanitized) stainless "straw" into the bunghole, with the tip pushed in just deep enough to touch the foamy krausen. The output end has a plastic tube running to the collection vessel, such as an Erlenmyer flask, fitted with one of those orange carboy caps with the 2 holes in it. The tube connects to one hole, with a plain old airlock on the other hole.
The pressure from the fermenting beer pushes the yeast and foam up the stainless steel "straw," through the tubing, and into the flask. The CO2 pressure then vents out the airlock atop the flask.