To cite a source in-text, use superscript numerals that correspond to the item in the list of works cited. If you're citing more than one source, don't put a space between the comma and the next reference. The examples below show how this works in the context of different kinds of punctuation:
As previous studies indicate,15,19
Previous studies have validated this speculation.4,7
Previous studies show this to be the case16-18; however...
Author AA. Title of article. Title of Journal [abbreviated]. Year of article;volume(issue);page numbers.
Holve S. An observational study of the association of fluoride varnish applied during well child visits and the prevention of early childhood caries in American Indian children. Matern Child Health J. 2008;12:s64-s67. [Note: in this example, "American Indian" gets capitalized because the phrase is capitalized ordinarily. "s64-s67" means that this article appeared in a journal supplement.]
An article with several authors:
Zagarins SE, Ronnenberg AG, Gehlbach SH, Lin R, Bertone-Johnson ER. Are existing measures of overall diet quality associated with peak bone mass in young premenopausal women? J Hum Nutr Diet. 2012;25;172-179.
For an article with more than six authors, list the first three authors and end that part of the citation with "et al":
Musa CV, Mancini A, Alfieri A, et al. Four novel UCP3 gene variants associated with childhood obesity: Effect on fatty acid oxidation and on prevention of triglyceride storage. Int J Obes (Lond). 2012;36;207-217. [Note how in this example, the first word of the subtitle ("Effect") gets capitalized.]
If your source doesn't provide an abbreviated title for the journal, look the journal up in NLM Catalog to find out how it should be abbreviated.
For an article in a freely available online journal:
Petersen JW, Nazir TF, Lee L, Garvan CS, Karimi A. Speckle tracking echocardiography-determined measures of global and regional left ventricular function correlate with functional capacity in patients with and without preserved ejection fraction. Cardiovasc Ultrasound [serial online]. June 14, 2013;doc 1. Available at http://www.cardiovascularultrasound.com/content/11/1/20. Accessed June 27, 2013. ["doc 1" stands for "document 1" - we're using that because this journal doesn't have regular page numbers.]
For book chapters, you can follow a pattern similar to citing journal articles:
Author AA. Title of chapter. In: [Author BB, eds.] Book Title. 2nd ed. Place of publication: Publisher; Year:pages.
Pastel RH, Ritchie EC. Mitigation of psychological effects of weapons of mass destruction. In: Ritchie EL, Watson PJ, Friedman MJ, eds. Interventions Following Mass Violence and Disasters: Strategies for Mental Health Practice. New York, NY: Guilford Press; 2006:300-318.
Author AA [if there is one; otherwise, use the name of the agency or group responsible for the web site]. Title of page. Available at: URL. Accessed [date you accessed it].
Centers For Disease Control and Prevention. Health effects of cigarette smoking. Available at: http://www.cdc.gov/tobacco/data_statistics/
fact_sheets/health_effects/effects_cig_smoking/. Accessed June 27, 2013.
The web site's navigation posed significant usability problems for study participants (A. B. Jones [firstname.lastname@example.org], e-mail, June 28, 2013).
Follow a similar format for other personal communications (e.g. interviews):
According to study participant A. B. Jones (written communication, June 2013)...
According to study participant A .B. Jones (oral communication, June 2013)...
To cite a YouTube video:
Author AA (or if unknown, video title). Video title (if not already named) [Video]. Publication date. Access date.
How to Perform the Heimlich Maneuver (Abdominal Thrusts) [Video]. YouTube. Available at: http://www.youtube.com/watch?v=tEIiEAn7b-U. Published May 27, 2009. Accessed July 9, 2013.