You are most likely to plagiarize when you are struggling to write a paper at the last minute.
To give yourself time for research and analysis, start thinking about the paper well before it is due. Weingarten Learning Resource Center can help you with time management.
Limit quotations and paraphrases to instances when they are really necessary.
The more you rely directly on the work of others, the more likely you are to accidentally plagiarize. Remember, research papers rely on but do not simply duplicate the work of others. An over-reliance on quotes or paraphrases when they are unnecessary could suggest that you do not understand the information well enough to synthesize it for yourself. Quotes or paraphrases are useful when another's work is being used as a primary resource, when you want to appeal to authority, or when you are summarizing.
Take careful notes and keep records of sources.
One easy way to do this is to use a citation tool that helps you as you go. For example, you can use PowerNotes or Zotero with ZotFile to keep a working bibliography and easily distinguish between quotations and your own notes.
When in doubt -- cite.