A great way to improve the overall quality of your beer and to ensure a great fermentation is to use a yeast starter.
If you would like to improve the process even further it is possible to enhance the yeast starter using one of a few different methods.
As simple as it sounds, swirling a yeast starter every couple of hours will actually help you to grow more yeast. As the yeast starter sits still everything begins to settle to the bottom. When yeast piles up at the bottom of the flask it can not consume the sugars in the wort effectively, thus it can not multiply and strengthen. When you swirl the solution the yeast returns to suspension and can work more efficiently.
The only thing better than swirling your yeast starter regularly is swirling it constantly. This can be done with a stir plate. You can pick up a stir plate at just about any home brewing supply store. It is simply a platform that magnetically spins a little bar, which you place inside the flask with the yeast starter. As the bar spins it creates a vortex which constantly swirls the entire yeast starter. Stir plates can seem expensive for having a limited task, but the service they provide improves the output of your yeast greatly.
A great way to help yeast proliferate is to add oxygen to the yeast starter. This can be done by shaking the flask fairly vigorously or even by setting up a diffusion stone to disperse oxygen into the beer. This setup can be tricky to pull off in a controlled yeast starter environment, and it brings other challenges. When oxygen is vigorously introduced into fermenting beer it produces diacetyl, which causes an off-flavor in the beer.
If you use this method of enhancing your yeast starter it will be necessary to refrigerate the starter for several hours to encourage the yeast to settle out. The liquid is then decanted off of the settled yeast, leaving only enough liquid to use in suspending the yeast so that it can be poured out. Since swirling the yeast starter using a stir plate introduces some oxygen to the wort anyway, you might find it easier to just use that method instead.
This one is simple: no matter what type of yeast you are using, let the yeast starter incubate at temperatures of 75 to 80 degrees F to encourage yeast growth. Yes, this temperature works even if you are using a lager yeast that is generally kept at lower temperatures.
These days there is some debate about whether it is necessary to add yeast nutrients to a starter. Since liquid yeasts are produced to be quite robust and are packaged with nutrients already, it is possible that adding extra nutrients will not in fact be used by the already-satisfied yeast, but will instead be used by other organisms which will then grow and possibly cause off-flavors in the beer. Even so, many brewers still encourage the use of just a pinch of yeast nutrient for yeast starters. Often, the recommended quantity is just 1/8 of a teaspoon per pint of yeast starter. This enhances the viability of the yeast and speeds up fermentation.
Great Yeast Starter Information
An excellent video about yeast starters by Northern Brewer:
Have you tried any other methods of enhancing your yeast starters? What has worked well for you?