A few hints that I have found useful.
When facing tanks much heavier than you, load HE ammo. Although the round wont penetrate the armor, the explosion from the HE has an increased chance of taking out components and crew. Get enough critical hits on a tank, and even the largest is soon near useless.
Where you aim at on a tank makes a difference. Hits to the turrent are more likely to take out the main gun, turrent or gunner/commander. Hits to the front may take out the periscope, driver/loader. Sides and rear shots will hit the tracks, fuel tanks, engines and often start fires or even cause an ammo rack explosion destroying the tank in one shot.
Be aware of which tanks have better hull armor vs turret armor and vice versa. For example, US heavy tanks usually have massive turret armor, but fairly like hull armor. If you can find a position behind a rock or other wrecked tank to protect your hull you greatly increase your surviveablility. I once took out 4 same tier tanks while in a T-29 doing that very thing.
Train your crews in camo as soon as they hit their 100% mark, and learn to use bushes and trees as cover. There is a practice mode in WoT that will allow you and a friend to go in and experiment on this kind of thing, so you can learn what hides you versus what reveals you and at what distances it makes a difference. There is nothing sweeter than being able to shoot indiscriminately at an enemy that cant see you.
Learn the maps. Most of them will have 3 approaches - a left, a mid, and a right approach. Learn which ones are more favorable to the type of tank you are using.
Unless you are a scout, never go out alone. You will just get picked off. Use the mini-map a lot to get an idea of your position relative to everyone else. If you learn to play smart, you will win more games. More wins equals more credits and more experience.