As you probably know, a plane needs two hits to go down. The first hit makes it smoke. The second hit will bring it down, and light it up on flame. The guy who land the SECOND hit get the credits. So, always try to shoot a plane at least twice in a row.
About tanks, they need five hits. I didn't realise yet how many shots it takes before smoking, but it doesn't matter at all. Tanks need five hits to die. Again, the guy who makes the fifth shot gets the credit.
So, if you see a plane smoking, hit him/her; but if you see a plane in flames, just LET IT BE, GO NEXT TARGET! Don't waste your bullets on a flaming plane! If he/she is taking some time to go down, he/she is lagging, so you better run away, cuz YOU WONT KILL HIM/HER (cuz he/she is already dead), and he/she CAN HIT YOU because of the lag.
To keep it simple:
If you read on the screen that you hit a plane twice or more and your victim doesnt go down, go next target IMMEDIATELY!!! Don't keep shooting at him! Don't even complain about it ingame, cuz the more you type, the less you fly.
The same goes for tanks: if you hit a tank five times and he doesn't die, GO NEXT TARGET!!!!