1) From Purdue University's Online Writing Lab (OWL):
2) Emphasizing resources such as YouTube videos, blog posts, etc. (From Trinity College's Cite Source):
3) Complete guide, with particularly good suggestions for formatting (from the Sierra College Writing Center):
4) Quick guide for basic citation questions (from Williams College):
Digital Object Identifiers are required in APA Style when they are available for online articles.
From A DOI Primer at the APA Style Blog:
"To search for DOIs, use CrossRef. The free DOI lookup searches for DOIs using information such as article title, authors, and publisher information. Or cut and paste your entire reference list into the Simple Text Query form and CrossRef will return all available DOIs at once." [The Simple Text Query form is free, but requires users to register.]
The official APA Style Blog provides many useful tips and suggestions for documenting sources. The archives are searchable, and the database covers many topics that are more heavily emphasized in APA than other styles.