Butting the panels tight. When aesthetics are less of a concern, butting the panels together is probably acceptable. To help improve appearance, take care to line the fasteners up straight, and be very careful not to over drive the fasteners.
Using batten strips to cover the fasteners. If you are going for a cleaner look, one in which the fasteners are covered, try batten strips. The battens lie over the fasteners completely concealing them. Battens can be arranged horizontally and vertically to your liking.
Using some type of joiner, such as a metal extrusion. Joiners can be used to both express the joints and to conceal the fasteners, but they only cover the fasteners at the edges of the panels. Joiners made of extruded metal are common in applications with a modern aesthetic.