"The best luck of all is the luck you make for yourself."
I think I have seen more battles won and lost as a result of poor deployment than for any other single reason. Deployment sets the stage for the whole battle, and as such, deserves prehaps more attention than any other single aspect of the game.
The first approach many people have to deployment is to think of it as merely a method to putting their army on the table, prior to commencing the fun. But, unless your opponent is doing the same thing, you're putting a large part of winning or losing in the dice roll for the first turn. If I wanted to win or lose based on one die roll, I'd go to Vegas and play craps.
As a trip to Vegas really doesn't fit my schedule, I think that there are three main things to consider when deploying. These are, in order, managing first turn risk, deploying with a plan, and deploying with the enemy's plan in mind. There are some secondary concerns as well, such as winning deployment. For most of this discussion, I will be assuming that games are played with at least a quarter of the table covered in terrain, as suggested in the 4th edition rules.
Managing First-Turn Risk
"Take calculated risks."
I think there must be a time in everyone's early Warhammer 40k experience where they've set their troops up, they're ready to fight, and they lose the roll for first turn. They then proceed to lose half (or more) of their models before they even get to move. Well, if that's going to happen, why not just take half the points initially, and ask if you can go first in exchange. I know if someone asked me if they could take an army half the size of mine in exchange for going first, I'd accept in a heartbeat. Why did this happen? Because of poor deployment that did not succesfully manage first-turn risk.
So the first lesson to learn is how to manage first-turn risk. If done correctly, the die roll for the first turn becomes far less of an ultimate deciding factor, and even going second can have significant advantages.
To deploy, managing first-turn risk is to deploy in such a fashion that your opponent cannot score easy points off you. Notice, however, that I have chosen to call this managing first-turn risk, not eliminating first turn risk. For without risk, there is no reward. Certain units cannot deploy effectively without the risk of being targeted themselves. Consider a heavy weapon squad. If it deploys in such a manner that the opponent cannot draw line of sight to it, then it cannot draw line of sight to any opponent. A heavy weapon team that is not firing is largely a waste of points. So, some of these units must be exposed to at least some risk, lest they become simply useless.
Vehicles, however, an almost always move and fire. So unless you have either a really cunning plan, or you know that your opponent lacks weapons that can hurt your vehicle, there aren't a lot of good reasons to deploy vehicles where your opponent can draw line-of-sight to them. Cunning plans? Well, you could be deploying with knowledge that a vehicle blocks line-of-sight to the units behind it, accepting the risk that the vehicle gets destroyed in exchange for ensuring the survival of the other unit. This is another situation where risk is being managed, rather than eliminated.
Other risks to manage include being assaulted on the first turn. A shooty army that falls prey to a first-turn assault is in deep trouble, as their firing lanes are likely compromised as a result. Fortunately, there are very few units in the game that can assault on the first turn if you deploy correctly.
The first mistake people make here is in assuming that when the mission's description states that they may deploy up to 12 (or 18) inches from their table edge, that means that their units must deploy at the 12 inch line. If you know you are facing a fast assault force, deploying at the back of your deployment zone ensures that the enemy cannot cross no-man's land and assault you on the first turn.
Once you've addressed fast-moving assault troops, the second concern is fast-moving infiltrating troops. Paying attention to the description of infiltrate will eliminate much of this possibility, as without line-of-sight, an infiltrating unit must start further than 12 inches from your units. That means that any unit without some enhanced movement capability will not be assaulting straight from their infiltration location.
But what of infiltrating units that do have enhanced assault capabilities? Well, the rules for infiltration force the infiltrators to start further than 18 inches away from any of your troops if any of your troops can draw a line-of-sight to the infiltrators. If you deploy your other units in such a fashion that at least one unit can see all the most desirable infiltration locations, you deny those locations to your opponent, and they must start further than 18 inches away. This is the best you can hope for, and, in most cases, it is enough.
Deploying with a Plan
"The highest generalship is to compel the enemy to disperse his army,
and then to concentrate superior force against each fraction in turn."
As you deploy your units, you should keep in mind your plans, both for defeating your opponent's army, and for achieving any mission objectives. Your plan to send a unit of scarabs to engage your opponent's shooter guys becomes useless if there are six feet of table between the scarabs and the shooter guys. Likewise, if the mission is to hold table quarters, and your troops are all deployed hugging the very back of your deployment zone, you will be hard-pressed to contest any quarters, let alone hold them yourself.
Sometimes, your plan for defeating the enemy will be countered by the requirements of the mission. You may have the perfect army designed for fighting tyrannids, with plenty of heavy bolters, and the perfect plan to simply stand and shoot, but when the mission comes up instructing you to go and seek out loot counters, either you adapt your original plan, or you hope your opponent will ignore the objective too. In cases like these, it is important to make this decision during deployment. If you still want to sit and fire, it doesn't make any sense to deploy closer to the possible loot counters at the expense of firing lanes or the possibility of being assaulted early.
Deploying with the Enemy's Plan in-mind
"No plan survives contact with the enemy."
To successfuly manage both your first-turn risk and set your plan in motion you must also anticipate your opponent's plan. Are they going to sit and shoot at you? If so, you will want to deploy in such a fashion that there is plenty of terrain to hide behind, not only on the first turn, but as you advance across the field. Are they going to send fast assault troops at you? Perhaps deploying at the very edge of your allowable deployment zone isn't the best idea.
What I think is the second most common deployment mistake (after not managing first-turn risk correctly) is making a mistake in figuring out which army is more mobile, and which is more massive. I think this is such an interesting concept that there is a separate page devoted entirely to it. So read on.
This is an odd concept, but it is based around the idea that if you have more units than your opponent, you can place your later units after seeing exactly how all their units are deployed.
As an example, we can examine two ork armies. One has a unit of three Killer Kans, a unit of three Warbuggies, a warboss, and two mobs of 30 boys each. The second army has three units of one Killer Kan each, three units of one warbuggy each, a warboss, and six units of 10 orks boyz each.
Which army is going to win deployment? They both have the exact same model count, and even the same models. But, the second army will still have 40 boyz, three buggies, and a warboss to deploy after the first army is completely on the table. This gives a significant advantage to the second army, as they have more opportunities to deploy in accordance with their own plans, and more opportunities to deploy in order to defeat their opponent's plans.
The extreme example of this is the all-infiltrating army (or, all deep-striking army), in which the infiltrating army gets to defer deploying any of their units until their opponent is completely on the table. However, infiltration is an ability, and therefore costs points, whereas simply taking more, smaller units provides flexibility relatively inexpensively.