Editing Warhammer 40,000/9th Edition Tactics (section)

====The basics====
#Shoot at something until it is DEAD/destroyed/exploded/gone/**cked/smeared-across-the-floor. Seriously. Anything that is not dead can still hurt you. As the old Tactica Imperialis says: "Concentrate your fire on one target to the exclusion of all else. Once it is gone, choose your next target." Consider heavily before you commit several turns worth of shooting into a big target like a Knight with your lascannons, you might be better off ignoring the Knight and targeting a Leman Russ instead if you cannot kill the Knight in one turn.
#Shoot for the mission. At the end of the game each game is won based on Victory Points (VP), don't fall for the distraction carnifex. 
#Shoot your weapons with the highest Damage characteristic first, if you deal a wound to a unit of Terminators with your Tactical Squad's bolters leaving one Terminator at one wound and then fire your Overcharged plasma into that squad with Damage two you are wasting 1 damage. If you had shot your plasma first you would get an extra damage for free. The opposite situation exists, if your opponent has a wounded model in a unit it can make sense to try and remove that model with your lower damage weapons in order to get maximum value of your bigger weapons into the next model in the unit. 
#Shoot with the unit with the fewest targets available first. At the start of your shooting phase, assess what target options each one of your shooty units has, and always have your units who only have a single option fire first. The reason for this is if you've got unit A in a position to shoot anything he likes and unit B which only has one thing to shoot at, you want B to take the shot first. If you shoot B's target with A first instead, you run the risk of destroying it or breaking their morale, and you'll feel like a complete moron for depriving B of anything to shoot at.