The most common method would be to blow in high density cellulose between the rafters. This method requires that a small hole (1 to 2 inches each) be drilled in sheetrock, at the top and bottom of each bay. After the insulation is blown in, repair the holes or use plugs to cover the holes. Foam can also be used, but is about 4X the cost of cellulose and very tricky to install without bowing or damaging the rock.
Regardless if you use cellulose or foam, this is not typically a DIY project. Many insulation contractors offer this service and have the right tools to assure a good fill.