Shoot down every flying thing you can down, except civilian and govt ship (Or eat the malus for doing that).
Especially the ratmens (Very slow craft, but stay for a long time one the map).
Of course, you need an armed craft (Aircar, Daredevil, or even land craft like the convoy or the battle cab !) for that, a weapon AND ammo (Most likely, 14mm, but LMG or HMG work against ratmens). So you need to plan for that.
Don't forget brainers, cause they help you get new shinny weapons, and also give you score (And money).
Also, try to unlock missions as soon as possible. More missions = more opportunity for loot / score.