I have worked in flight test engineering in the past and am a certified multi instructor.   I like Capt Mos explanation of the weight effect.  There are actually seven factors that we teach new multi drivers to reduce VMC. 1) Higher AC weight 2) Lower density altitude 3) Landing gear down 4) Flaps down  5) Lower power setting 6) CG located forward in the envelope and 7) Prop feathered on the sick engine.

1) Mo explained well.  2) Lower DenAlt gives better control at lower speeds due to higher dynamic loads on the control surfaces 3) & 4) Give additional lateral stability 5) Is the obvious P factor and torque issue 6) With the CG forward the rudder has a longer lever arm through which to create anti-adverse-yaw moment.  and 7) With the prop feathered, there is less pro-adverse yaw moment created in the direction of the sick engine. 

Of course some of these factors which reduce VMC might be inadvisable, given the aircraft situation at the time; but, if you are in a corner and need additional control of the aircraft, these will get it for you.  Remember, it is better to maintain control of the airplane even in a challenging situation.