A 3-button mouse provides quick access to SketchUp's most frequently used viewing tools: Zoom, Orbit and Pan. Learning to use a 3-button mouse is the best and easiest way to speed up your modeling.
Zoom - Scroll the wheel to Zoom in or out. Use your cursor to choose the part of your model you zoom to.
Orbit - Press the scroll wheel like a button and hold as you move your mouse to Orbit your model.
Pan -
While Orbiting, hold shift on your keyboard to Pan