What wears out a clutch?
Slipping — the period when the clutch plate and flywheel are rotating at different speeds while in contact. That is when friction material is being ground away, and every technique that extends it shortens clutch life.
How a clutch works. A friction plate is pressed against the engine's flywheel by a spring-loaded pressure plate. Fully engaged, they rotate together with no wear. Fully disengaged, they are separated with no contact. Wear happens only in between, and heat is what destroys the material — a badly slipped clutch can reach temperatures that glaze or burn it in minutes.
What causes excessive slipping:
Riding the clutch — resting a foot on the pedal, applying just enough pressure to partially disengage. This produces continuous low-level slip and is one of the worst habits.
Holding on a hill with the clutch instead of the handbrake. The clutch is slipping continuously against the full weight of the car, generating a great deal of heat. Use the handbrake.
Slow, drawn-out engagement. Releasing over several seconds rather than smoothly extends the slip phase every time.
Excessive revving during engagement, which increases the speed differential and therefore the wear.
Pulling away in too high a gear, requiring prolonged slip to get moving.
Towing or carrying heavy loads, which increases the work the clutch does.
Sitting in traffic with the clutch down wears the release bearing rather than the friction plate — a different component with its own failure mode. Neutral and handbrake is better.
Signs of a worn clutch: the biting point rising toward the top of the pedal travel; engine revs climbing without corresponding acceleration, especially uphill or in high gear; a burning smell; and difficulty selecting gears.
Life expectancy varies enormously with use — 60,000 miles of city driving may wear a clutch that would survive 150,000 on motorways.