The BEST way is to not let them get there in the first place. Once someone is on your six, even if you fly perfectly you may still die. Though, If you have a significant turn performance advantage, simple hard turns will often be enough to avoid them.
Basic guns defense involves staying out of plane with the person trying to shoot you. Avoid simple flat turns, go slightly up, or slightly down, dont stay in the same turning plane.
As for getting people off your six, It mostly comes down to making them screw up, or having a wingman ready to clear your six.
If they're going fast, you can make small side-to-side turns, and try and force either an overshoot or a scissors. A short blacked out turn followed by a barrel roll can work on occasion. If you're flying an F4U of any kind, throwing down the landing gear will give you an added speed brake. If you have altitude, a defensive spiral can sometimes work. (It's basicly a rolling scissors going straight down)
All of these though, come down to forcing them to overshoot. You need to be slower than your pursuer, or able to slow down much faster. If you're not, you'll just make yourself a very easy target.
The only reliable way to clear your six is to have a friend nearby who will do it for you. (Don't expect help from people who havn't commited to flying with you) To do this, you need to drag your pursuer. Avoid high-G turns, or any rapid direction change. Stay out-of-phase with your pursuer(That is to say try and keep your wings perpindicular to his) Small random direction changes will make you a hard target, and hopefully keep you alive long enough for your friend to clear your six.
The worst thing you can do with someone chasing your pursuer is to make hard turns. It allows your pursuer to close the distance, and get a gunshot, while making it as hard as possible for your friend to kill him.
Disclaimer: this may be entirely incoherent or flat out wrong, I take no responsibility for any k/d ratios ruined by following this advice.