Clutch slipped, won't repeat

My clutch slipped once for me, once for the mechanic. I didn’t get the repair immediately, but I can’t get it to slip again. Any ideas why? Should I just drive it until it disintegrates? Or should I assume the clutch is on it’s way out and replace it immediately? It’s a 94 Ford Escort Wagon. 310 thousand miles, I’d guess about 80 thousand on the current clutch.

When they start slipping they usually do so only under high load conditions. In 4th gear floored while climbing a hill, for example. If you just keep driving it it will slip more and more until one day you’ll be going down the road at 40 mph in 4th gear with the engine at redline…and the smell of burnt popcorn.