I'm sitting with my converegence set at 275, and seldom try for a real kill-shot beyond D200. I may try to "steer" my opponent with a few sprinklers beyond that though. I'd say the majority of my kills occur on the first instance I pull the trigger, rather than several firing passes later.
F4U with .50's (of course)
I'm basically running at a 9.9/1 Kill to assist ratio if you average my last 6 months.
I'd recommend making sure you're close enough, firing at convergence, and able to land a sustained burst on the bad guy.
I also will almost always simply fly past/ignore a bad guy that has been or is in a fight. If he turns for me, or if I get asked to help then I'll shoot him, but I'm normally looking for a "fresh" plane to fight. If I end up attacking an enemy after he's fought, it's almost invariably after the friendly dies, so any damage he's racked up won't count against me anyway. I often suspect I'll get an assist before I actually see the message if I'm violating that pattern.
As a result, 99% of assists for me come as a result of not killing the guy on my first pass, and someone else cleaning him up for me. Getting close, firiing at convergence, and landing a sustained burst on a plane eliminates that for the most part. If the guy you shoot explodes, you're kill can't be "stolen" either...