Common mistakes
Ducking does not confer any bonus in a combat, it just allows you to cower behind cover like, well, a coward. Furthermore, changing state from standing to ducking will actually make your unit incapable of defending itself for a fraction of a second. This is often long enough for your unit to lose a vital body part, like a head or torso.
Partial line of sight does not provide a defensive bonus, I.E., peeking around a corner will not make you harder to hit. Cover bonuses are only granted when standing behind half height (light blue) objects.