When being in the gunner seat of a chopper:
1) Spot things, whoever is driving will usually be looking front, it's your job to go "Hey look! AA! Run for it!" or go "Hm...that Tank sure looks like it needs a TV Missile."
2) Most idiot chopper pilots will charge right into a enemy copter, a smart one will float a bit higher and let you TV it, recognize this chance to bag 2 kills (or 1)
3) If the enemy copter doesn't see where you are yet, don't use your gun.
4) (Does not apply to MEC) Since your gun does no splash damage and require direct hits to injure the enemy, blaze away! If you see a something that you can't TV missile, shoot at it, shoot at it until it goes out of range or it explodes. Particularly infantry, even if you miss, the infantry will start running far away.
4a) (MEC piece) Your gun is the only thing that does decent splash damage in flying in the air, exploit this advantage by not just spraying and praying. If you are going to infantry behind a wall, don't hit the cover, hit BESIDE it, over ir or the other wall if there's another one behind the cover, the splash damage will take care of them nicely.
5) Leading a target: Sometimes, you will see a speeding Humvee, Vodnik or DPV speed past, shoot at it, shoot about 1 meter (depends on vehicle speed and your speed) in front of it and it will literally drive into your bullets. Or at least it will prompt it to stop and shoot at you, or panic and crash.