The front of the clutch (nearest to the rad) has a small "spring". This is heat sensative (like the old style mechanical thermometers) and turns as the heat increases. As it turns, it turns the innnerts of the clutch and tightens it.
Typically an always loose clutch is due to a broken spring.
Wonder is that little spring can be bought somewhere? hmm.

Reminds me, I need to replace my clutch too :)

If you're worried of overheating and have no $ you can always somehow lock the clutch... at the expense of some noise and performence..

Greg Mustang