One key skill is to understand how OP teamwork is. If you see a teammate is struggling, if you can safely help, go help. 2 guns are better than 1. You never want to give the enemy a fair fight. If your team mate is a 1 shot and you can save him by taking a hit, take the hit. Keeping your guns on the field is crucial.
You have to think of your Hit Points as a currency. You spend it to gain advantage in the game. If you need to "spend" some HP to get into a better position, take a hit or two. If you need to save a team mate, take a hit or two. if you need to break a stalemate make a push and spend some HP. If you have 100% HP at the end of a game, you basically didn't contribute to your best capability.
It is a careful balancing act though. If you make a mistake early in the battle and lose 90% of your HP, it is better to withdraw to safety and play at the back, than to keep playing aggressively and sacrifice yourself early in the game.