I have fun harvesting yeast from bottles.
One thing I have found that helps, is to use the slurry from multiple bottles of the same beer. Last summer I did a really successful Hoegaarden harvest from 12 bottles. What I would do is after pouring the beer into a glass, I would spray the lip of the bottle with starsan and then re-cap with a sanitized cap. Then store in the fridge until I had enough bottles. Then on harvest day I would spray the caps again with starsan, open them, flame the lip of the bottles, and re-spray with sanitizer, then pour the dregs into my starter.
I ended up with 2 good sized mason jars with yeast. It's made some really tasty wits since then.
This is a pretty good list of beers with harvestable yeasts, though it is waay incomplete and hasn't been updated. It's still a good source.
http://www.nada.kth.se/~alun/Beer/Bottle-Yeasts//