I leave gaps in my line when i'm fighting Hellenistic factions, it confuses their phalanx and lets me hit their flanks from the front line while sending the Velites up the side.
For barbarians, I close the front line up defensive. When the Hastati get tired, I move the Principe up where I want the line to be, and simultaniously order the Principe (not in defensive) to throw their Pillum and have the Hastati run back through them behind the Triarii. By the time the Hastati are through, the Principe are charging a usually a broken enemy. After the Principe are spent just retreat them through my defensive Triarii and have rested Hastati watch the flanks (by this time the Velites are sword fodder).