When wind forces place stress on walls, it's important that the walls be properly anchored to the foundation. A registered design professional, an engineer or an architect can determine if these joints need retrofitting, which can be done by a qualified contractor. For houses with more than one story, the connections between each level must be properly tied together. These precautions can improve the odds of your home surviving high winds, but they won't make it tornado-proof. Nor are they a guarantee of personal safety.
Tornadoes can form and move quickly, so plan ahead.
When a tornado threatens, take shelter immediately. The more walls between you and the outside, the better.