id say a good tip when your confronted at medium to close range is to CROUCH mid fire... the amount of times ive been up against someone at close range and im shooting at them then suddenly their hwad is 3 fooft lower that where you were aiming !!
i soon learnt to crouch when shooting, it really helps to loose the other guys aim, and ofcourse increases your accuracy too!
its been siad before but SOUND in this game is a MUST, i mean really you can hear more than you can see !
when your at spawn, your at perfect headshot hight if you dont move the mouse, it feels SO damn uncomfortabel as it kinda feels liek your head is tilted back, but get over that feeling and you will own with the headshots !... so in a nutshell, learn key heights in every map where head hight is, most maps give you big clues to work off, office is perhaps the easiest to judge head height.
never aim at peoples heads while theyre ruunning along, by the time your finger has reacted theyre 1 metre tot he left of right of where yourr shooting ! always aim just a little infront of their heads
lastly STAY CALM, and take your time... if it means your gettign owned over and over again for not being quick enough i dont think this matters if your aiming headshots,a nd taking your time and gettign good solid Genuine headshots, not just by fluke, then just speed that process up and youl be ownign in no time !!
Rick