With ard boyz coming around, its time to say once again. Play the freakin mission. Yes, it seems simple. I myself, am the worst at this. I generally play 1-3 rounds of kill my opponent before I even think of the mission. This is a mistake. Play the mission from round 1.
You should be playing the mission first. Everything else is secondary. Ive played many games of 40k, where Ive literally decimated the enemy, and yet either come away with a minor victory, or even a draw. Its important from the start of round 1 to have the objectives very clear in your mind.
Here is what I suggest. At the start of each round. Take a 1 minute pause. Take one step back from the table. Look at the table. Remember the objective of the game. Take your turn. If you do this, you will know exactly what you are doing, and the round should make up for that one minute pause.
Annihilation missions. What can I destroy this round to get a kill point. If the mission gives more kill points for troops or hq's as in last years ard boyz missions, figure out where these are and how to kill them. Each round you should be getting points. Make it happen. Target priority are efficient and complete kills every round.
Objective missions. How can I reach the objectives. How can I stop my opponent from doing the same. Ask yourself those two things every round. Knocking out mobility of troop units in an opposing army if vital in these missions. In fact, all other enemy units that are not troops should be thought of only as obstacles; they are the things that need to be dealt with that will prevent you from claiming your objectives. So target priorities are enemy objective claiming units (troops), anything mobile that can contest an objective, and things that can or will prevent you from claiming and holding the objective.