Probably the most common cause of a clutch replacement is running a clutch out of adjustment. As the clutch discs wear, the throw out bearing assembly moves forward, and eventually begins to press against the clutch release fork.
Eventually, the fork will actually prevent the clutch from fully engaging, increasing the chance of the clutch discs slipping. Once the discs beging slipping, the resulting heat quickly deteriorates the clutch friction material.
Including a regular and frequent clutch pedal freeplay check in your maintenance schedule will go a long way to ensure long clutch life. Also, show your operators how to check the clutch pedal freeplay themselves, as they may notice a hard clutch pedal before you do.