1. Concentration of Troops, Concentration of Fire
The Guard cannot afford to place units in isolated positions, or to spread them out across the length of a table. Its greatest strength is its mass of firepower, and since most of its guns are 24" Rapid-Fire weapons, that means that the way to get the most use of your units is to concentrate everything (or, at least, the units that won't be moving much) into a small area, optimally no more than one-half your deployment zone.
The primary reason you would want to concentrate your army is very straightforward; not only does this maximize the effectiveness of your heavy weapons, but it also means that more of your army will benefit from the special Morale-enhancing rules of your officer corps. In other words, you want all your Heavy Weapons to be able to shoot at the same enemy target(s) at once, instead of being scattered about, and you want all your units to be within 12" of your officers (particularly your senior officer) if at all possible, to take advantage of their excellent leadership.
You should concentrate the guardsmen that are IN your units, for similar reasons. If one member is in range, you want EVERY member of the squad to be in range. Furthermore, if you are assaulted, having lots of supporting attacks nearby will help you do better than usually expected (note that this is only a marginal improvement; Imperial Guard are not very good at assaults one way or another). Accordingly, there are two types of formation, tight and loose, and I will discuss them now:
Tight Formation offers a number of advantages. First, it allows units to screen units behind extremely effectively. Your opponent will need to kill over half the unit before the units behind begin to become exposed. Second, it offers virtually every member of the unit the opportunity to make attacks in an assault, while limiting the number of models an opponent can get into base contact (simply by having less space around each Guard base). Finally, it is a compromise between the first advantage, which favors a straight line, and the second advantage, which would favor a vaguely circular cluster. It even stands up very well to most enemy guns.
In Tight Formation, the actual placement of special and heavy weapons is up to you, but typically the longer-ranged heavy weapons (and the high-strength special weapons) get more protection in the back row. Flamers should probably be placed in the front (especially if you expect the unit not to come under a great deal of firepower right away), as most opponents won't let you flame "through" your own people. The flamer is suggested on the far left or right (to get a cross-angle at any units assaulting), and sergeants, should you equip them with a laspistol and CCW, should be placed in the middle of the front row. If the unit comes under a great deal of fire, he can be the first removed (as his laspistol has the least range), and if the unit comes under assault, he is most likely to get his extra attack if he is base-to-base contact in the midst of a melee.
Loose Formation is the formation you wish to adopt if you are facing enemies with any of the following types of weapons:
(a) any army using even a few units with fast-moving or long-ranged flamer templates, or
(b) even one unit with a direct-fire ordnance template with sufficient range to your lines.
Furthermore, a unit in the open that faces (c) barrage weaponry (mortars, Eldar support guns, guess-range ordnance) or (d) Plasma Missiles or Plasma Cannons (and their Dark Eldar equivalent, the Disintegrator) should also organize in such a manner.
A unit in tight formation will wither if faced with any weapons like these, and so a looser formation is required, with models spaced out between an inch and a half and a full two inches away from each other (measured from base to base). If you do this, you will not only need to place your unit in a zig-zag formation like above (in order to prevent enemies from shooting "through" your unit as much as possible), but for maximum protection of screened units further back, must place another screening unit behind the first, to cover all the "holes" in the protection.
Loose Formation should be seen as a distasteful compromise, and a less-effective alternative to Tight Formation. Note that most weapons will hurt both types of formations equally effectively (or not, as the case might be with, for example, bolters and lascannons and autocannon and the like), and Missile Launchers are not as effective as you might think. They will cover only about 5 targets in Tight formation (remember that template is only 3 inches wide), and because Guardmen get armor saves against Flak missiles, they will on average kill only 2 (possibly three) each time, assuming they hit. Those are acceptable casualties for a Guard commander, comparable to being shot by a Heavy Bolter. As for indirect-fire Guess weapons, they are inherently inaccurate, and if you are in cover, you can expect to survive the worst of it in Tight Formation even if the Guess is accurate and the weapon does not scatter. Plus, a scatter or bad guess is more likely to miss a unit in Tight Formation.
Recall that the Doctrine Close-Order Drill provides additional benefits to units where all models are in base-to-base contact. Since you will prefer setting up units in this fashion, it's a useful Doctrine to have, giving your screening units (most importantly) a +1 Initiative bonus while they remain in that formation, helpful when they inevitably get assaulted by I4 opponents. It will give your infantry a slightly better chance to do some damage before they go down for the count.