Basically, you need to know you are being attacked before you can defend against it. This requires good SA (Situational Awareness). Be on the lookout for any dots higher than you. Watch relative motion for clues to the direction of the dot's travel. Don't depend too much on radar to give you warning. Radar has a finite limit for range and if you are on that edge, you can see the plane before it shows up on radar.
Once you know you are being attacked, you must escape or engage. To escape, try diving away from the attacker, diving toward friendlies, or diving toward an ack region. You want to discourage the attacker from blowing their E advantage for little or no gain.
If you are going to engage, you must not wait long to do it. This is particularly true if the bandit is in your rear hemisphere. A good rule of thumb is never to let them get closer than 2000 yds behind you and I personally think that even that is to close in today's arena.
When the timing is right perform a break turn to make the bandits job of aiming harder. I personally prefer to turn some, roll some more and then turn some more. This really puts you out of the bandits plane of maneuver and makes their job difficult.
If you time it right, you can actually complete your turn as they overshoot and you now have a chance to shoot them. They must then commit to extend or engage.