I got my invite yesterday, and I'll share the secret - there's no secret. Follow the Ingress Google+ page. Make up something artistic (photoshop or real world), post with #ingressinvite tag, ask nicely, repeat many times. After 6-7 pieces of "art", including a few photoshops, some hand-sketches, and a woodcarving, it was dropping 9 pieces of firewood in the triangle-in-hexagon logo pattern in my driveway and snapping a photo with my phone that finally did it. It must have been quantity, because it sure as hell wasn't quality.