How much difference is there between a Great player and an average one?
The answer may surprise you. A WOT player named LesterQuaestor put up some graphs showing a great player, a bad player and an average player. A great need only influence 7 games out of 100. A bad player only needs to blow 3 games out of 100. So his point is that since you never know when these games will happen, you should try to win every game regardless of how bad it looks at the start or how bad it gets in the middle. Please read his excellent post here:
You have all seen them...base campers. You know like the guy in the KV1 in a Tier 5 battle who is going to "defend base". What he's really going to do is keep his gun out of the battle until it has already been won or lost. Then especially if its a loss, he will do damage to the other team before he gets swarmed. Looking at the Battle Results screen, he will see he is in the top 5 and think "I did my share, the rest of the team sucked!" In reality, he contributed to the loss. The damage he farmed was "Loser Damage", meaning it could not contribute to a win. Games are won by doing damage early and often. Don't sit in the back and wait for tanks to come to you, even if you are in a low armor TD. Make a difference. Don't be "that guy".
Win a side, defend base, win the other side.
It's really that simple, but many people don't do it. Most maps, there are 2 sides or corridors or flanks. Once you pick one, count tanks. If you have more than they do, you have an overmatch. Be aggressive and help your team win that side. If you have less, defend it.
If you win the side you chose, don't push to the enemy cap, instead go back and defend yours. If it doesn't need defending or you have defended it, then go help win the other side.
Disclaimer: This is general advice. There are situations when you should cap, but take a mental note of how many people die pushing to or sitting on the enemy cap in your games. You will notice that it is a bad idea more often than not.
Scenario: You are facing an enemy tank. You peek/poke from cover and get shot. You say "OH CRAP!" and pull back into cover.
Time and time again, I see this scenario. You already messed up and took damage. Pull forward, make an aimed shot and deal damage, then pull back into cover.
Obviously, this does not apply in some situations: 1) Against an autoloader, 2) Against a tank with a fast reload, 3) When there may be other enemies to shoot at you, etc.