Your voice acts as an alarm to attract help and to catch your assailant off guard. It's good to practice yelling as loudly and fully as you can, by opening up your windpipe and letting it go. Your throat opens up and the back of your tongue lies pressed to the floor of your mouth when pronouncing a long "O" sound. Emulate the position of your tongue and throat during an "O" sound when yelling. This opens up your throat to produce a fuller sound. A good loud "No!" is like a slap in the face to someone who isn't expecting it. Using your full voice also expresses confidence, strength, a strong will, and a reluctance to give up easily. You might be tempted to tell the assailant that you know self-defense. However, this might be interpreted as more of a challenge than a warning, and negates any element of surprise, so try avoiding this. A simple "leave me alone!" would be more effective.