I've stolen the yeast out of many bottle-conditioned beers. You'll need:
-The beer you want to steal the yeast from.
-A airlock and stopper that will fit the bottle you're stealing from.
-A 1-gallon (Approximate) fermentation vessel.
-Some dried malt extract (or pre-prepared wort).
1. Prepare about a gallon of simple wort using the malt extract. You'll use a bit of this in step 4 below - and the rest in the subsequent steps.
2. Decant the beer from the bottle you want to steal the yeast from into a glass. Do it VERY slowly and in one, long motion (i.e. I one continuous pour).
3. STOP pouring when there's about 1/2" to 1" still left of the beer and/or when you start to see ANY sediment start to leave the very bottom of the bottle.
4. Fill the bottle about 1/2 way up with the malt extract you've prepared (try to have the wort at proper fermentation temp).
5. Attach the stopper w/ airlock to the bottle and wait about 3-4 days. Don't worry if you don't see a lot of 'activity' in the bottle. Often the small amount of yeast that's been re-activated and multiplied is too small to detect just looking at the bottle.
6. After 3-4 days, fill the 1- gallon fermentation vessel (I just use a 1-gallon glass jar) about 1/2 way with more of the simple wort you prepared. Then swirl the contents of the bottle you're stealing from and pour it all into the gallon fermenter. Attatch airlock.
7. In 2-3 days, you should have enough trub at the bottom of the 1-gallon vessel to either pitch or use as a starter if you want to multiply it more before use.
I hope this simple guide helps. Good luck!
EDIT: I see ryang beat me to the punch. W/ his info & mine you should hopefully be good to go.