Thanks to Bob Yorburg for his kind words.  Bob's mention of an
idler arm is worth considering if the belt is correctly sitting
in the pulley and the system is still on the verge of slipping.

One of those equations I still remember from my university days is

  T1/T2 = e power (mu*theta)

What this means is that the amount of torque that can be transmitted
by the belt is proportional the power of the angle that the belt sits
around the pulley.  Adding an idler wheel such that the belt rotates
further around the pulley before leaving will greatly increase the
driving capacity.  In the above equation:

  T1 = tension in belt arriving at pulley (driving pulley assumed)
  T2 = tension in belt leaving pulley
  e = mathematical constant approximately equal to 2.718
  mu = effective coefficient of friction, as previously discussed
  theta = angle of belt wrapped around pulley (arc of contact)

I hope some people will find this useful.

Best regards,
Nicholas Simons, UK

 [ Equations about pulleys and maximum torque are presented at
 [ http://www.codecogs.com/reference/engineering/the_theory__of__machines/belt_and_rope_drives_brakes.php 
 [ -- Robbie