How to stuff and/or spice the chicken BELOW the skin?
It's a tradeoff: salting (and using herbs or spices) a chicken under the skin, rather than on the surface of the skin, will result in a better-seasoned bird but it will make the presentation of the whole bird a bit less attractive, since the skin is no longer exactly where it should be. I've also noticed that the skin gets crispier when you salt it between the skin and the meat. 

You can use a spatula as BaffledCook says but I find it easier to just use my hands. Just find a spot on the bird's breast where the skin and meat are a little separated, like the chest cavity opening, where it's been cut during butchering. Slowly slide your fingertips in between the skin and the meat, separating the two, and work your way all over the breasts with both hands. Go slow so you don't tear the skin. If possible, leave the skin attached at the breastbone and near the wings, that will keep the skin relatively in-place for presentation and there isn't much meat that needs seasoning in those spots anyway.

You can work your way over to the thighs and drumsticks this way too, though if you do, the skin will be quite separated from the carcass and your presentation will suffer more. Also, thighs and drumsticks aren't quite as thick as the breast, so salting them on the surface of the skin is more effective.