I've never figured out how to get rub under the skin. Once it gets moist, it's hard to position it. Had a bright idea this morning that worked pretty slick.
Cut thin slices off a stick of butter. Lay them on a piece of wax paper and throw them in the freezer for 30 minutes or so. While they're freezing, shake some of your rub into a shallow bowl. Take the butter slices out of the freeze and shake them around in the bowl of rub. Pull the skin up on the bird and slide the slices of rubbed butter where you want them.
Works great! I don't figure the butter hurts the chicken any... might even add some moisture and help crisp the skin? I figure that as it melts, the rub floats around a little and spreads itself.
All I know for sure is that the birds turned out great, the rub's flavoring was much more pronounced through the bird, and the skin was golden brown and almost crisp.