Keep on the move and avoiding enemy fire. Keep in mind you need to get the bad guys in your sights otherwise you won't shoot anything.

Always kill the ones who are firing the most rounds, or the most damaging rounds first.

Don't fly over the mines even that way they'll take a chunk of damage.

Save enough to get a new helicopter, because you can't upgrade your existing one's armor.

Keep your cash to buy the new weapons it's far better than upgrading.