What I usually do is I go into an instant respawn deathmatch server, and I unbind my reload button. That means you do not reload until you are out of bullets. I know it sounds silly, but the adrenaline rush you get when you have 5 bullets left and 3 enemies pop into B on dust2 as you come through tunnels can help you in getting those 3 headshots and not dying. And even if you do miss, it is practice for pulling out the pistol to save your life. It's just a good way to learn to conserve ammo, get the headshots, reduce spray, weapon switch, and deal with high tension and clutch situations. Another thing I used to use in CSS was my spray -- which was a crosshair with multiple dots in a circular pattern, in which I did a multitude of aiming warmups before a deathmatch. However, due to CSGO not allowing sprays, I've had to discontinue this practice.