Hold the middle button and drag

Flies in or out, or spins

Drag down to fly out, and drag up to fly in. Drag right to spin clockwise, left to spin counterclockwise.

Hold the middle and right mouse button, and drag

Slides the camera across the model

Press SHIFT+hold the middle mouse button and drag

Flies up or down, or spins

Drag up to fly up, and drag down to fly down. Drag right to spin clockwise, left to spin counterclockwise.

Press CTRL+hold the middle mouse button and drag

Orbits the camera around the model