In a multipalyer game when you are in a 1 on 1 siuation both with SMGs for example. When your HUD is red and flashing and your about to die get into the habit of throwing a grenade at your opponent. Aim at his feet so it makes a big contact with him.
Because your opponent is probably close to death your grenade (if well placed) is almost garaunteed to take him out and get you a kill.
Even if you die at least you get a kill out of it. There comes a stage when this becomes natural instinct but try to get into the habit as early as possible.