Best advice I can give is this: Take your time. After you decide on a
move, stop. Check to make you sure you aren't moving something
into danger.
Think: "If I were the opponent, how would I respond to this move?"
If it seems like the opponent has a good move in response, consider
making a different move.
In this format, you have plenty of time to look things over-- get used
to doing it slowly, and over time you will be able to do it faster, and in
face-to-face play.
For starters, that will help a lot. All the strategy in the world doesn't
make a difference when you go and throw pieces away. Believe me, I
should know. 🙂