The first step to achieving a good fermentation is to pitch enough yeast. The yeast can be grown via yeast starters or it can be harvested from previous fermentations. When yeast is harvested from a previous fermentation, it should be taken from the primary yeast cake and preferably from the upper layer of the cake or from the secondary. This yeast will have the optimum characteristics for re-pitching. In either case, you should target pitching at least 1/3 cup (75 ml) of yeast slurry to a typical 5 gallon batch of ale or 2/3 cup of slurry for lagers. For stronger beers, OG > 1.050, more yeast should be pitched to ensure optimum fermentations. For very strong beers like doppelbocks and barleywines, at least 1 cup of slurry should be pitched.
The yeast that is obtained from a healthy starter or recently from a prior fermentation will have good vitality and adapt readily to the new wort. With good levels of aeration and nutrients, the yeast will quickly multiply to the numbers necessary for an exemplary fermentation.