Use the loci method. Break down your speech into paragraphs or bullet points. Visualize a picture for each bullet point (like imagining a Harry Potter if you're talking about J.K. Rowling's influence on children's literature). Determine a location for each of the points (like Hogwarts for Rowling, a meadow for Stephenie Meyer, etc.). Now you'll progress through the locations (you fly on a broomstick from Hogwarts to the meadow, for example). If you have multiple things to say about each specific point, then put them in specific places around the location (like a point about Harry Potter's popularity in the Main Hall, or the effect she had on revamping the genre in the Quidditch field).