What he said.
Here's some good reading on starters: http://www.mrmalty.com/starter_faq.htm
Basically, you want to prepare a starter of about 1.040 gravity. As far as how big of a starter to prepare, there's a great calculator at http://www.mrmalty.com/calc/calc.html
that helps you determine the optimum starter size given your beer's style and OG.
Both links provided are from the website of Jamil Zaniasheff, who's a very well known and respected home brewer.