This article is from the Beer and Other Brewing FAQ, by Kurt Swanson email@example.com with numerous contributions by others.
The Wyeast package recommends making a 1.020 SG wort and pitching
the active contents of the package into a sanitized bottle with
an airlock to allow the quantity of active yeast cells to build
up before pitching into a typical 5 gallon batch of wort. This
"starter" wort is usually made from dry malt extract boiled with
water at the rate of 2 tablespoons per 8 oz. cup of water. Some
brewers like to throw in a couple of hop cones or pellets for their
antiseptic qualities. When the starter is at high krauesen (the
term is used loosely here, you often won't get a foamy head on your
starter, look for visible, strong fermentation) it's ready to pitch.
Typical time for a starter is 24 hours. This technique is recommended
for both dry and liquid yeasts.