Retrieving yeast from commercial live beers

I've heard that you can culture the yeast from the bottles of living beers. How is this done? Is it worth the trouble?

Some folks seem to have success and it can be done with some stepping up of starters. Personally I don't think I would take the trouble to often but may one day for experimentation.

Another thread on the subject.

Yes you can get yeast from bottles. You should sanitize the tops of the bottles before you start to uncap and just after and then decant off the beer so just a tablespoon is left. Swirl the bottle and pour the contents into a sanitized mason jar. Do this with 2 more bottles so you now have 3 times the yeast. Add 10 ML of boiled and cooled wort to this. The wort should be at 70F before mixing it into the yeast. Step this up to 100 ML in a larger container that is sanitized. Step up again to 1000 ML (1 liter). You ferment at each step until the Krausen just falls. You only need foil over the mason jar, and do not use the screw on lid. Everything you need to know is in the article below.

Ref: MB Raines, Ph.D. - Guide to Yeast Culturing for Homebrewers - Maltose Falcons Home Brewing Society (Los Angeles Homebrewing)