The easiest way to increase your win % would be to platoon capable/carry tanks with players of the same or better skill level. Play off of one another, set up crossfires when possible and focus out dangerous or inconvenient tanks when possible. Other than that it just comes down to a few general things; positioning, trading your HP effectively and being able to judge the flow of a battle.
Now a lot of this just comes with time, feeling out where the favorable positions are on each map and what tanks can excel in those positions is a nice start, you probably already have a few of these spots. Think about what kind of tanks you are playing and if you can carry with that tank or not, play to its strengths.
When going for shots, think about how many guns are looking at you, don't want to lose all that yummy HP in one peek. Get to know tank reloads. Don't get lazy or greedy when going for shots... people can get lucky and snap you.
Knowing when and how to push a flank is hard. If you are trying to go for wins you will need to be leading the charge a lot of the times (pubbies are too scared). If your team is going to lemming you need to either crush that flank asap so you can return to base or put pressure on theirs or, you need to be in a position to slow the enemy down enough to stall for your team.