What is the correct technique for working on an airframe with a pneumatic rivet gun?
I banged rivets on a production line in a previous life (in the 70s). A trick that works wonders (although it may be frowned upon by purists, and your school instructors may prevent you from using such a "cheat" while learning) until you get good at holding it perfectly perpendicular is to put a little piece of masking tape over the tip of the rivet snap.  It moves the snap a few thousandths of an inch away so that more misalignment is tolerated as well as putting a barrier between the edge of the snap and the skin surface if it does touch.  You will find your smiley rate goes way down.

I tried to draw a little picture of what I meant in Paint but I couldn't do it without it looking like a rude bathroom stall drawing, if you know what I mean...

The other thing I see people doing is just shooting it full blast, then off.  Don't do that.  The gun has a "feathering" trigger that lets you ease in the air for a reason.  When you start to set the rivet, ease on just a bit of air, just enough to get it ratatatatting, and feel for the feedback of the bucking bar on the other side to tell you you have good contact, then give it a full shot.

Each shot should sound like, "ssstat tat tat tat tat tat brrrrrrrrrrrrrrrrrrrrrrrrr". 

Especially when you have a partner bucking, you'll know right away if they don't have the bar in place on the shop side.  This helps prevent depressing the metal around the rivet, as well as reducing the risk of smileys.

Another one is don't hold the rivet gun by the handle like a pistol.  You have to be able to push in line with the snap.  Old time rivet guns were made U shaped for this purpose, but all the modern ones have the terrible pistol shape.  Move your hand as high as possible so you are pushing with the heel of your hand on the back of the rivet gun's body, with your wrist more or less in line with the snap.  Your index finger will be too high, so use the middle or second last finger to work the trigger. Like this: