Deployment is 90% of your game. Ok, maybe it is not quite that high, but looking back, how many people have lost games here. Deployment is a big part of game and if you fail here, you face a completely uphill battle and in fact the game may already be over at this point. What I tell newer players to do is, practice your deployment. It is the easiest part of the game to practice at, becuase it takes no opponent to do so. Simply set your army up against various army types and terrain types. Go into a game with a deployment strategy ready to go. Here are some things to think about during deployment.
Moral, simply put, dont play your opponents game. Dont fail a player moral check before the game gets even starts, it will force you to make mistakes. You can always cry later.
Army types, know what you are facing and what their capabilities are. That way you can deploy accordingly and take advantage of your strengths vs your opponent. You can often put your opponent on the defensive at deployment.
Terrain, Dont be afraid to get down and look at line of sight on the table. Actually look at the table, where area terrain might be, and how to best take advantage of it. We all do this at the most basic level, but only a few opponents have I seen actually get down at los level, and look. Especially if you are setting up second.
Armies with pre-game tricks often happen at deployment. Eldar can sometimes re-deploy some units, storm troopers must pick a special operations type, red thirst for blood angels, and callidus assassin's can move an opponents unit 6". These are things to be aware of, and things not to forget if you have them.
Infiltration is the next step. It is a great under utilized tool by most new or newer players. A single infiltrating unit, can often times, completely halt or slow a scout army's advantage before the game. If your opponent has scout abilities in their army, its important that your infiltration be set up to limit their scout movement. Vendettas and Valkyrie's have a big movement range, as do Baal predators, scout bikes and a host of other scout moving units. You will need to infiltrate to reduce these units movement plans.
Infiltration is also a great tool to respond to your opponents set up if you go first, and can put you in forward positions on the field. Taking a mid-field ruin can make you very tough to remove, and give you great board control during the game. If you are playing against an infiltrating army, dont forget what and where they can go. Set up to reduce infiltrators pre-game advantage. Meaning, set up to push them out of that mid field ruin.
Scouts units are a great way to strike first and hard. If you are going first, it is not hard to drop melta bombs directly into armored lines, effectively reducing them to smoldering wrecks. Baal predators, and Valkryies are some of the most common on the field. These will be moving quickly to forward positions, to unleash very quickly in the game. Be aware of what scouting units will be doing when facing them, whether its flamestorm cannons, or melta guns and melta bombs. Know that scouting units can be in your face quickly. They are a great tool for taking forward positions and flanking your opponent, for side armor shots, or taking ground.