Hi there!
Most people choose a sample of 30 to ensure that the sample means are normally distributed.
References:
http://www.isixsigma.com/library/content/c030506a.asp
http://people.richland.edu/james/lecture/m170/ch07-clt.html
http://people.hofstra.edu/Stefan_Waner/Realworld/finitetopic1/sampldistr.html
Let me know if that's what you were looking for...
-Scott
MIT Graduate
College degree in math... proficient in all levels -- from algebra to calculus