Talk to the audience about the character or about the situation in which he finds himself. Often, the other characters freeze while the character who is speaking to the audience has all the attention. He then moves back into the original position and continues the scene.
Change your body mannerisms when you break character to address the audience. This will help indicate that a transition from the scene. For example, a police officer might have a strong posture, but underneath he is a calm, laid-back person; when addressing the audience, assume the calm, laid-back posture and tone.
Face the audience to give them your full attention. Often during a presentational delivery, a spotlight will focus on the actor with the rest of the cast and scenery set in black so that the audience isn't distracted.
Gesture towards the audience in an exaggerated manner. For example, when you speak to its members, use your hands to grab their attentions. Use your head to also gesture to the audience by moving it towards them to emphasize specific points.
Address the audience in a similar fashion each time so they know they are being addressed. For example, sit in a rocking chair each time you address the audience, while reading a book and smoking a pipe.