Try and mix up the angles so it's not all of the same thing. In a Cessna 172 I mount it where the windshield wraps towards the door under the air vent with a RAM suction mount so it can easily be swivelled to show a a forward view and side view into the cockpit.
Obviously you'd pic a safe, level flight, moment to swivel the camera (if you have a passenger put it on their side).
I did a few circuits with the camera in different positions and edited it to look like one lap from the two different point of view. Also allows for edit points when you want to cut footage
The free GoPro Studio software is great.
Another thing is try and keep videos brief. One way to do this is speed up bits such as the taxi or a long straight and level leg where there's no extraordinary scenery. It also ads something different to the vision to keep the viewer interested.
With sound it's good to have engine noise, though I put some music in the background - which is good when you speed up the vision as you lose the audio.
I look forward to finally hooking up audio to the Go-Pro so I can include radio chatter. I have the right cables but just need a skeleton frame so i can access the mini USP socket on the side of the camera.
As for lens angles you can change them electronically in Settings, though I'm not sure how much it varies.
Oh yeah, if you're going to be in shot make sure you shave ;-)