Before the Shooting Phase Begins
Optimizing your Shooting Phase doesn't begin when the phase begins, it begins at the beginning of the turn, before you even move any models. You need to know what your goals for the turn are before you start moving models. And, to do that, you need to make yourself a mental list of what your targets are, and what your resources are.
Target Priority is the first thing to consider. What is important this turn, and what can be ignored? This list will change over the course of a game. On turn 1 of an objective mission, the highest priority target might be whatever enemy unit can deal the most damage. On turn 5, however, the highest priority target might be the lone guardsman who is sitting on an objective, minding his own business. The best way to establish a Target Priority list is to ask yourself, what can this unit do that stops me from winning. The more that a unit can do to stop you from winning, the higher up the list it should be.
In addition to your priority targets, there will generally be opportunity targets. These seem to fall into two categories. First, there are high payoff shots. This might be a unit with multiple flamers close to a bunch of clumped up infantry, or it might be a twin-linked meltagun within 6" of an empty rhino. The common theme here is that it is a shot that is highly likely to cause a lot of damage to your opponent. The second type of opportunity target is a shot that is unlikely to do much but really has nowhere else to be directed.
Reasons Not To Shoot
Take some time to consider if there are reasons that you might not want to shoot at a given target. The most common reason will be that you want to assault a target, and the charge range is pretty close, such that if you killed a model, your opponent could prevent you from charging by removing the model that is in range.
The last thing to consider, before moving, is how effective your shots are likely to be. An enemy Land Raider full of Assault Terminators may seem like a juicy target, but if all your marines have to fire at it are a couple of krak missiles, you have basically no chance of destroying it and it might be a better use of that squad to fire at something they can actually kill. Generally, you should avoid shooting at a multi-wound model unless you have no other viable targets or you are relatively certain you will kill it this turn or at the very least next turn. If you have 12 lascannons in your army, you shoot the first four into a Castellan and make it use the Rotate Ion Shields Stratagem giving it a 3+ Invulnerable save against shooting attacks, you are probably better off trying to kill any other vehicles in your opponent's army as the remaining 8 lascannons are unlikely to kill the Castellan. Don't get tricked by the sunk cost fallacy, even if something has lost several wounds it might not currently be the best target if you will be unable to kill it, removing a minor threat immediately is often a better way to victory than setting up for the removal of a bigger threat three turns down the road, instead try to play around the units you cannot kill, by engaging shooting units in close combat or blocking the movement of melee units with your less important or more durable units.
Moving with Shooting In Mind
So, now you have your list of priority targets, as well as knowledge of any high payoff opportunity targets. At this point, you start to maneuver your forces in such a way that you have the highest payoff potential for your shots.
During the Shooting Phase
Order of Shooting
The order that shots are taken in is another opportunity for a skilled player to gain additional effectiveness out of their shooting phase. In general, the following guidelines will give you a more productive shooting phase. Sometimes, two of the guidelines will contradict each other, in which case, you'll need to weigh the pros and cons based on the situation. The overriding concept is to maintain flexibility throughout the Shooting Phase, and into the Assault Phase. The more options you leave yourself as you go through your shots, the more successful you are likely to be.
Fire at high priority targets before targets of opportunity. Opportunity shots are just that, they're an opportunity to deal more damage. But, if the priorities aren't taken care of first, you stand to lose the game.
Fire weapons that have only one possible target before weapons with multiple possible targets. If your meltagun kills the tank, your lascannon has enough range to take a shot at a target of opportunity. If your lascannon kills the tank, your meltagun will not do anything that turn.
Don't waste more fire on a target than you need to. Many units are vulnerable to morale, the last two Guardsmen in a squad of 10 will automatically run, you don't need to shoot them unless they are holding an objective and you fear your opponent might use the Stratagem to automatically pass the Morale test.
Shoot at loaded transports early enough in the shooting phase that if they are destroyed, you will be able to shoot at the unit that is forced to disembark as well.