Induction Motor Torque Slip Characteristics
slip per torque is proportional to magnetic field. so a weaker field will cause more slip.  (I'm not speaking accurately it might be a square relationship etc.)

Most motors are already operated with the stator near magnetic saturation (else there would be excess iron in the stator) so it's not practical to increase the magnetic field.

one approach that may pay off is to use a different type of motor - the permanent magnet types do not slip in normal operation.

another approach would be to use a larger motor or to add another motor possibly coupling to back shaft of the existing motor.

another would be the replace aluminium in the rotor with a better conductor like copper or silver. 
As Nail Tosun comments cooling the rotor also reduces its resistance and thus reduces slippage a little.