The best article I’ve read about how to calculate power and sample sizes is Russell V. Lenth’s “Some Practical Guidelines for Effective Sample Size Determination” in The American Statistician (full reference below). It is written for statistical consultants who assist researchers who need to make sample size estimates, so it is just a bit on the technical side. Since it is so well written, however, I recommend that clients read it as well. Whether you are working with a statistician who will do the calculations for you, or you are doing them yourself, the article details the information you need to gather and what you need to understand about the calculations.
Researchers who are at a university that subscribes to JSTOR can read the article for free here (for all others it is $14).
Dr. Lenth’s Power and Sample-size web page has an applet that calculates power and sample size, more information about calculating power and sample sizes, and a version of the paper:
Lenth, R. V. (2001), “Some Practical Guidelines for Effective Sample Size Determination,” The American Statistician, 55, 187-193.
This recommendation is listed in the Statistically Speaking membership program Resource Library.