For him: Go all out with blue body paint! However, you may want to get a full skirt as opposed to the Na'vi's teeny-tiny loin cloth. Attach a makeshift tail in the back, but don't forget the most important part: your Na'vi braid! You'll want to buy a wig long enough to tie into a ponytail.
For her: Buy a blue leotard and leggings and draw on some Na'vi stripes. Then strategically cover up your important bits in front and back (a tan/blue half apron might work in a pinch). If you have longer hair, braid a few strands with beads and feathers.