I use a big mouth bubbler to ferment, and I skim the top kreusen off the beer when fermentation starts to slow, before it drops. Use a well sanitized ladle or large spoon to skim off the yeast, a little beer too doesn't hurt. put it in a sanitized mason jar, and stick it in the fridge. What's cool about this, is that the yeast you save has the exact same characteristics of the parent yeast culture.
Some yeasts are really good to do this (White Labs California, Burton Ale, many Belgian ale yeasts). I've used three generations (2 skims) of yeast with no problems, but haven't risked more than that. I usually do a starter also, but if you use the yeast in a couple months, and if you collected significantly more than you would get in a White Labs flask, you simply take out of the fridge to let warm up to room temp, and pitch that yeast directly.