It is a great way to save a little money and get a really good pitch rate, so here are a couple of options to get you started:
The easiest way to reuse the yeast is to just pour your new beer onto a yeast cake from a batch you just moved to secondary. This of course requires you to have a beer ready to ferment at the same time you are ready to move a previous beer to secondary. This technique also creates a HUGE pitch rate, which can actually lead to off flavors, or what can be referred to as being “too clean”, meaning there is no esters at all and makes for a strange flavored ale. Use this technique mainly when lagering, where you need a lot of yeast and are trying for a super clean flavor anyway. A popular use is going from a light lager, such as a pilsner, to a big alcohol lager like a bock or double bock.
The second easiest way is to pour or scoop a quart (or so) of your yeast cake from a finished batch into a growler, mason jar, or whatever you can easily sanitize and use for collection. This technique will allow you to put the airlocked container in a refrigerator and use it up to 10 days later without any issues. You can however go longer with this technique, but much like the liquid yeast you get at your home brew supply store, the cell count diminishes over time. You should be able to do a yeast starter within about a 6 month period of packaging, but your changes diminish over time.
-- Always move from lighter colored (less hoppy) beers to darker, so your yeast doesn't impart any color (flavor) to the new beer.
-- Only use yeast from primary fermentation any time after fermentation has dropped your O.G in half.
-- Do not use yeast from secondary fermentation because they are fatigued and contain mutations that you don't want to start in another beer.-- Only reusing your yeast to many times can also lead to a variant strain of yeast, that might be good, but possibly not as good as the original strain, so try and limit your reuses to 6 or less.