ACS
The American Chemical Society (ACS) has advice and guidelines for beginning researchers on writing undergraduate abstracts.
AMA
The American Medical Association (AMA) explains how to prepare an abstract in this video.
APA
The American Psychological Association (APA) provides a guide for creating an abstract, plus keywords.
Chicago
The Chicago Manual of Style provides information on writing, formatting, and citing abstracts.
CSE
For Council of Scientific Editors (CSE) Style, California State University San Marcos (CSUSM) has an excellent example format of a scientific paper, including abstract and references in citation-sequence, citation-name, and name-year.
IEEE
IEEE has comprehensive help on writing all aspects of a paper, including abstracts.
MLA
The Modern Language Association (MLA) offers an example of how to write an abstract.