Install additional attic insulation at right angles to the previous layer. You don't have to use the same type of insulation - it's fine to use batts or blankets over loose-fill, or vice versa. Upgrading from three inches to 12 inches can cut heating costs by 20 percent, and cooling costs by 10 percent.
Toenailing styles can vary. Some pros like to drive three or four nails into a stud, toenailing from both sides at about a 30-degree angle. The job is easier if you first drive a holding nail on one side of the stud, then drive two nails on the opposite side. Remove the holding nail, then toenail the second side. You can also make up a 2×4 spacer to place between the studs.