Know your tank's strengths and weaknesses. Speed, agility, firepower, armor, turret rotation speed, weapon reload time -- all of these are extremely important as they will affect how well your tank performs.
Know the other tanks. This will take time, but it's very helpful to be aware of the key characteristics of the other tanks you will face. For example, if I'm in a T-34 1940, I will think twice before going up against a Tiger, at least by myself. Another example: I chased and destroyed a KV-2 with my Pz IV F2 because I knew the KV-2 has a reload time of ~42 seconds. That's an eternity in this game (heck, 5 seconds is an eternity), and I knew I would have time to kill him.
Be aware of actions that can give away your position, like firing, knocking down trees, and moving. Yes, you will need to move and fire, and occasionally knock down trees, but keep these actions to a minimum.
In realistic battles, try to avoid moving when enemy aircraft are overhead. They will have a harder time seeing you if you're stationary. The exception is if you're out in a field or something and you're standing out against the grass or whatever. You need to find cover fast in that case.
Some locations that appear to be good hiding places, aren't. In realistic battles, hiding in trees or brush can make it hard to be seen, but the risk is that an experienced player will see you first and pop you.