I would add composted manure not sand. You may increase drainage with the sand but only down until the less permeable layer. If you add too much you will create a perched water table and the water will perc too fast and potentially wash your nutrients through. It might behave something like a golf green where the nutrients aren't held much in the soil.

Organic matter will lighten and "fluff" your soil and add nutrients. Sand just adds permeability.