Chess Tactics: Counting
The single tactic that drastically improved my chess is Counting. This is simply counting points on pieces that if attacked and re-attacked will accrue. This is often key in openings and focussing on this will take away a lot of the naive pieces that you may give away up-front. Take a look at The Greatly Misunderstood and Potentially Challenging Tactic "Counting" to start you off.