1. Never defend a bomber from his close 6 co-alt. Your job as an escort is to eliminate threats before they get into guns range on the bombers. This cannot be done from a position within guns range.
There will be some argument on this but my opinion is that the best place from which to defend is the bombers high 6. At least 3-5K above them and ~3K behind them. This allows you to dive onto an enemy's 6 before they reach guns range. Even at that, timing is everything.
Others will argue that the high 12 is a better position but this is more an offensive than defensive position, better suited to clearing the skies ahead of the bombers than actually defending them.
2. Avoid reversing. Most buff hunters will come in from an altitude advantage with WEP on. This is why we prefer a high 6 escort position. If you have to reverse you give up too much E to stay with the attacker.
3. Never chase an extended attacker. After the first pass take position above and behind the bombers rather than trying to chase an extending attacker.
4. Don’t let ego and/or lust for a kill make you leave position. Never leave the remaining bomber(s) totally alone in enemy territory.
5. Do not engage enemy fighters who aren't a threat. Anytime you are engaged the bombers are on their own. You're job is to eliminate the threat to the bombers as quickly as possible and then move back into a protective position.
Eliminating the threat doesn't mean you have to kill the attacker, just drive him away. Even an attacker who is extending away ahead of the bombers is better dealt with from the high 6 defensive position as he will have to reverse at least once, and probably twice to gain a guns solution leaving you with the E advantage.