In new construction poly sheeting is stapled to the frame before drywall is put up. That is ideal. In existing construction, poly sheeting can still be spread over the area to be insulated and pushed into the rafters. Alternative would be to use 15 or 23" strips to lay in between the rafters before blowing in insulation. I really like the rigid foam soffit vents sold by Owens Corning to maintain attic ventilation.
I think this site may help you: