My suggestion is to get to know air combat manoeuvring (ACM) better so that you are more proficient both defensively and offensively. The more time spent in a potentially offensive position the better. Being on the defense at range is a manageable situation but most don't recognize that they are in this situation until they are what is really a "guns defense" type situation where the enemy is behind them and in a firing position.
Wikipedia is actually a decent starting place:
http://en.wikipedia.org/wiki/Basic_fighter_maneuvers
Some of the key types of moves that I tend to employ:
Negative Knife Edge - Basically you roll 90 degrees until one wing is facing the sky and the other the ground. Normally the lift vector is now pointed towards the horizon in the direction of the top of your aircraft, however, this is where you push forward on the stick and go in the opposite direction that your plane would normally do. Its a good quick move when your opponent sees you rolling into a defensive turn and your motion takes you in the opposite direction. Timing is key as this is a deception to evade an attack rather than a solution in itself.
Rolling Scissors - Putting your plane into a barrel roll and slowing it down so that the attacking aircraft is now sucked into your roll is another way of defending from an attack. The idea is to fly tighter and go slower without loosing control so that he shoots out in front of you and you are now potentially in the attacking position.
Spit S - A simple one in the bag of tricks that works well on aircraft with high roll rates. A FW190 or a Yak is ideal here. A the opportune moment you roll inverted and dive away steeply.
Those are a few... there are others. As always, team tactics works best so a wingman backing you up is a great thing to have. Also the best defense is, as they say, a good offense. Being in a superior position either from a speed or altitude perspective (energy) is key to winning a fight. If you can dominate the fight from the beginning then the enemy will never have a guns solution that would even threaten you. That takes time, patience, and some luck.