I know not to attack them from behind. Someone told me something like: "make slashing attacks, rake your guns along their plane, wingtip to wingtip".
Follow the first part of this advice, but ignore the second half!
Deliberate raking the enemy bomber is just wasting ammunition. People that do this are the ones complaining about the bomber's seemingly unreal damage resistance "I put XXX round into him, and he still flies!"
You should always try to concentrate your fire. Of course, it's not always easy to to, but even in fast slashing attacks you should aim for a single point to ensure maximum concentration of your hitting power. Best places to aim at are cockpit (recommended), wingtips (hard to do in slashing attacks) and in case of the B-24 the wingroots. But even if you hot another location, it's always important to concentrate your fire - better break one vital part than just soften up a dozen different ones.
For setting up your attacks, it's basically all about
patience and
practice. Most fighter pilots fail, just because they don't take their time to get into a favorable attacking position. They climb/stalk a bomber for 10 mins, and then refuse to add another 2 mins to get into any other position than sticking up their nose into the bombers butt...
If you get above and ahead of the bomber, and then slash down to attack his 1-3 clock position (or 9-11), you are putting the enemy gunner at a disadvantage: Not only you are a fast moving target, but you are also coming in at very different angles that make you (combined with some lack of orientation for the gunner when shooting upwards) a very difficult target to track. It's seems to be more difficult for you to hot the bomber, but trust me: It's much more difficult for the bomber gunner.
Another way is the flat head-on attack, but I wouldn't really recommend it unless it simply happens to present itself: The high closure speed increases the collison risk, and you may find yourself in a bad position for setting up your next attack after passing the buffs.
Particularly in US fighters with many .50 cal guns and even more ammo, you may also try for a diving 6 oclock attack from high altitude. It's easier for you to hit, but at 500mph you are still a difficult target for the enemy gunner.
Unfortunately I can't put up some films tonight...maybe tomorrow.