Back on my old blog, the most-viewed entry was a short piece I wrote linking to instructions for "spotlighting" certain parts of an image on a slide. That method still works, but I found another one that is shorter by a few steps.
Alas, PowerPoint still has no way to do on-the-fly spotlighting during a slide show--you'll need an extra piece of software such as Presentation Assistant if you want to spotlight things "live."
Evidence in the research literature suggests improved learning when students/audience members are overtly directed to important features or content points. This is known as "signaling" or "cueing." In the example above, I've used the spotlight to direct the viewer's attention to two areas of significance, but you can also do it verbally: "Notice the menu of options on the left side of the screen." An example of signaling that might be more familiar to you is when a teacher tells his students: "This will be on the test."
I'll talk more about signaling in a future post, as I think it's a "secret weapon" that most presenters rarely use. Stay tuned for that!
In the meantime, give the technique a try, and credit for the new method I found goes to ShapeChef.