My experience is primarily from the point of view of a P38 driver.
I find the biggest factor is getting altitude above the bomber formation. If I have a big altitude advantage, I generally have good success, and my transit through an acceptable firing solution generally has deadly effect, while my transit through the areas where I'm vulnerable is very quick. If I'm clawing for altitude and energy, I am generally forced to spend MUCH more time in those areas where I'm vulnerable, and I generally get killed.
I prefer a diving attack into the bomber's path, head-on from their front, but from much higher up. However, because those bombers are generally moving fast and the dive itself takes several seconds, the angle often changes dramatically, and a straight-on 12:00 high result isn't commonplace for me (and isn't required).
So I would summarize my most successful attack angle as commencing from high up, somewhere in the bomber's forward quarter, but often ending up diving through the middle or rear of the advancing formation.
Some video clips showing successful attacks:
"Ten Brave Men":
http://www.youtube.com/watch?v=dubnqBCppCMAlso the first clip and the last of the four clips here:
http://techvideoreview.com/FlightSimMovies/AcesHigh/MissionMovies/FourClipsFeb2012Full.htm 