The only gold gun I got is the AN-94, and I'm not sure how I got the headshots for that thing. Probably mostly just out of luck.
Anyway, I normally get headshots through regular play, but now I want to focus on getting them deliberately. I feel that I have to hit them in the tip of their head otherwise it's a normal kill, but aiming carefully in the middle of a gunfight isn't exactly encouraged in COD.
Not removing this until Dr. Dre's Detox album is out. 11/22/08 THOU ART THEE ART THOU STICK THY HAND IN TV, DO IT DO IT DO IT DO IT DO IT!
Just aim high. That's what I do. Sure you won't get a headshot everytime but you'll notice you're getting them faster. Just like you said you don't really have time to aim when in a gun fight. Also you can catch the noobs who are away from their controller at their spawns for easy headshots.
Shock charges and stuns will slow your enemy down giving you a little time to line up a headshot... also try to flank around enemies allowing you to come up behind them for a free headshot on those campers.