Did your grandmother ever tell you to scrub behind your ears? If she did, she's about to be very pleased.
The skin behind the ears will be the location for two more of our reference electrodes.
Swab the skin over the area behind both ears (the skin over the skull, not over the ear itself).
Abrade the area, focusing particularly on the 'bone bump' if you can identify such a bump.
Given this is a difficult area to obtain a good signal, it may be worthwhile to spend extra time on the abrasion step.