I gotta completly disagree with the low attacks. They're good against planes with no belly protection, but only give you one good shot. Usually if you dont kill the bomber in that pass, you'll driift into a 6 o clock position and thats the end of you.
The best way to attack a bomber is high speed steep diving attacks from the 9, 11, 1 and 6 o clock positions. It gives the gunners a very difficult shot, and puts you in a good position to make multiple passes.
My personal favorite is to lead the plane, and fly a straight course(ideallly perpendicular to the bombers course) so the bomber will fly close(200 yards or closer) past my nose, with me passing him just behind it's tail. The lead can be difficult to judge, but it gives you a solid(if short) fiiring opportunity, while making it quite hard for the gunner to track you.