Few tips that personally helped me and probably some you already know or do but I'll post it anyways.
1) Know your gun's recoil
The gun's recoil will help in knowing where to aim your first shoot. A gun like the AN-94 has some vertical recoil so you unless your bursting it you'll generally want to aim in the chest area because it'll kick up to the head and get you the head shot. A gun like the M27 has practically no recoil so you pretty much want to aim at the head from the get go with a gun like that.
2) Play objective based game modes like Domination (or any mode that requires capturing or planting)
In a game mode like Domination people typical sit still and are focused on one direction to capture a point so it's easy to aim and shoot them if you approach them without them noticing you. Also helps that they usually are kneeling or prone which makes tagging the head easier.
Hardpoint is a good mode as well once since people pretty much do the same thing as they do in Domination
This one is subjective but personally I liked using the EOD sight a majority of the time since it gives a nice balance of being zoomed in but not too zoomed in which helped me a bit when it came to lining up headshots. ACOG to me was very gun based as some guns I loved the ACOG on and other guns I hated it but that is something that aid more in getting the long distance headshots. Red Dot Sight is fine as well.
Pretty self explanatory as the perk will enable to better stay on target and get that headshot while being shot at.
This one is mostly personal taste but 5) Silencer
The reduced muzzle flash can help you aim a little better and to go along with number 2 if you flank and get to the side or behind multiple capturing a point you can typically kill one without the other ones noticing so get the headshot with one and proceed to line up and get another headshot with the other one.
The flip side to this is the reduced range could cause you to miss out on a headshot that you normally would've gotten without the Silencer equipped.