What would be a good soil mix for a raised bed?

Raised bed soils should be light and well-drained.  An excellent soil mix can be prepared by mixing equal parts topsoil, organic matter (well-rotted manure, compost, or peat), and coarse sand.    If bringing topsoil from other location, ensure that the soil is not infested with soil-borne plant pathogens or contaminants like lead, pesticides, etc. Likewise, ensure that the compost is well done, mature and does not carry too much salt.

Before filling raised beds, remove the grass sod (if present) and work up the existing soil with a rototiller or spade.  Add a few inches of the soil mix, then incorporate it into the existing soil.  Continue to add and incorporate additional soil mix until the raised bed is filled.  Incorporating the soil mix into the existing soil prevents the formation of distinct layers in the raised beds.  Distinct layers of soil impede water movement and discourage root growth. 

When filling the bed, grade the soil so that it slopes slightly away from the center of the bed to the edge and away from adjacent structures. Use mulch around plants in raised beds to conserve moisture and to control weeds.