I keep any yeast strain that I like the flavour profile, ester production, alcohol tolerance, attenuation, flocculation, etc. Here is a link to the Wyeast recommendation http://www.wyeastlab.com/hbrew/hbyewash.htm
There have been numerous posts on the forum about how people do it but most are fairly similar to this or at least resemble this wyeast protocol. Just think clean, clean, clean storage vessel.
Good luck and you'll save yourself about 6 bucks a batch.