Some time ago Brulosopher posted a thread (which has since been replaced by an article - link below) about an easy way to "harvest" yeast. His "novel" approach is to make a bigger starter than called for and save a small jar's worth (~500ml) of the mixture (with the yeast still in suspension). Super easy. I'm writing to (a) tell you it works and (b) show you with a picture.
I bought a pack of Wyeast British Ale II (1098) on 11/1/12. I made a large starter of about 2.3L. I planned on brewing a few batches soon with the yeast so I poured about 500ml (each) of the starter (with yeast in suspension) into two small jars. Fast forward to the pic below... I still had both jars of extra yeast from 11/1/12. I began a starter with one small jar and 2L of ~1.040 wort (DME, aerated with O2). Bang. 5 hours later I have this monster. 36 hours later I have the two jars filled with yeast (one small and one large 1L jar). Check out the size of the cake in the 1L jar compared to a quarter. That's a ton of yeast.
Bottom line: Super easy. It works. My $7 pouch of yeast has made two 6 gallon batches and I have enough left for at least three more. This is NO MORE DIFFICULT than making the starter that you probably already make.
Kudos to Brulosopher for the ideal.