Winging as a rule is about give and take. And about knowing your partner. When I wing, as stated 2k or so is a good start.
In our game it's almost more about being good "bait". Your enemy is not going to want to tipically engage 2 enemy. So one of you needs to hang his butt out a bit ( bruv) can confirm I'm pretty good bait. If the enemy is focused on the prize in front of him he doesn't see the second one coming.
As for the wingman, always make sure you are " holding" 100+ mph on your leader when your not playing bait. This can be done by having altitude, ( and that might make it that you need to be more than 2 k away) but its ok if you are directly over the fight.
Always be thinking in your head (if my leader gets reversed on, in his current engagement can I clear him in under 30 seconds) also be watching the fight, does it look like your leader is losing his advantage? If so start getting your speed up now, (dive) if things go well, no harm, climb back up. If it does go south, your already diving in.
Talking to each other as stated befor is key until you have gotten to a point that talking is just silly. And what I mean by this is, if your going left right up down or backwards you need to let each other know. Your leader wants to know you are right over the fight and ready to clear. He also wants to know that you have 18 bad guys all within icon range 4 outside icon, 4 with alt, and 2 that could be trouble in the next few min.
You as wingman need to inform the leader of the big picture outside so he can stay focused on the prize in his gunsights. When his time is upand he needs to break off, warn him early " you got about 20 seconds to get him befor this D-9 comes in.
This gives him fair warning that not only does he need to start aiming for the cockpit, but that he needs to start looking for the 190.
It's tough to say everything in just a message but I think the big things have been covered.
Don't stay too close, keep the leader informed of all things that could kill you both, if your 2 on one and he doesn't wanna fight, someone needs to be bait.