Put quotation marks around copied material even in your notes. Then, when you begin to write your speech, you can avoid plagiarism by recognizing what you originally quoted.
Make a master list of sources. Keep very detailed notes that include the author, date, name of publication, page numbers and anything else that quickly allows you to reference your material and to double check for unintentional plagiarism.
Choose caution when you are not sure whether or not you should cite a source. If you are not sure, cite your source. It is better to be on the safe side.
Give credit to the original author when you paraphrase or use his ideas. Credit is not limited to direct quotes. One way is to use the term "quote" at the beginning of the quote and "unquote" at the end. However, this may not contribute to the flow or style of your passage. Another option is to cite your source, including the date, author and name of the article or book in a more free flowing manner. This is effective for both direct quotes and for paraphrasing.
Determine if what you want say is understood as having no known author, such as when you use a common saying or cliché. In this case you may leave out a citation.
Include complete authorship information for any borrowed audio clips or visuals you use in your presentation. Insert them into the presentation.
Reduce the number of quotes and paraphrases to only what is necessary; be original with your ideas.