Crosshair size: smallest one selectable in options
Crosshair colour: Red and opaque (not the translucent option.)
Aiming: Ideally you want to aim for the head all the time, but i usually tend to aim for the head in all situations except long range. At extremely long distances, without a sniper rifle I usually am not precise and quick enough to aim directly for the one pixel on my screen that is their head, so usually I shoot for the torso in 1-2 shot bursts. In really close range I go for the head almost always because its so much easier to aim for and I can aim for it much quicker. In fact in mid range, I can pretty much aim for the head in the same time it takes to aim for the torso anyway. If it is obscenely close, like they walk within an arms length in front of me, or if they walk into my crosshairs before I have even aimed, then usually I will begin spraying because I know some will hit and then try to adjust the spray pattern up toward their head, but usually you have to develop a feel for how that specific weapon sprays or else you end up shooting over them.
Spraying whilst running: This is a definite yes. While running no matter what gun in the game you cannot be precise. There is no sharp shooting while running, it is about luck IMO, thus the more bullets you send flying their direction the higher your chances of hitting them vs. just clicking off one or two shots. The idea with running though is that if you are already running, you want to run toward them, the closer to them you get, the more effective your shots are. Usually its a good idea to strafe side to side randomly while you run toward them. Sometimes I burst a few shots until I get closer just in the name of conserving ammo if I know im far enough away that I will run out of ammo before killing them. Or even sometimes I don't shoot at all and let them waste their ammo first until I get in closer.
If they don't see me though, then I don't run toward them and shoot and alert them of my presence, in that situation running usually isn't needed and you can just crouch and pick them off more precisely.
For running the five seven, glock, usp, dualies, sig, mac10, and TMP are absolute monsters. If I was running right towards the faces of 3 people I honestly think I would have better chances of headshotting all them quickly with a mac10 or TMP than I would an AK or M4.
Teamwork: A necessity.