The first and most crucial one has to be, buy Apocalypse Reload!
Compared to the Guard formations available in the main Apoc. rulebook, Reload contains far more goodness for most of us - I was able to jump from using 3 formations to 8 using only this new book.
'Imperial Shield' Infantry Company
This formation requires that an Imperial General use 1 Guard Command Platoon & 3 Guard Platoons and, if you can muster up the models, is well worth it. The formation grants you 3 strategic assets: On My Coordinates!, Scheduled Bombardment & Defence Lines. While the benefit of the free Bombardment is obvious, I had thought I would get less out the other two assets - I was proved wrong.
On My Coordinates! effectively allows you to place an Apoc. orbital bombardment directly on any Guard model with a Vox Caster or Master Vox, once per turn. As well as being really in keeping with the background of the IG (and thus awesome), I found it is combined well with the 'Armoured First' Mechanised Platoon, which I will describe later. The cost of a Vox Caster for most of your units is well worth the fear it creates with the enemy, who suddenly become unwilling to close with your lines. It also creates those cinematic situations were the plucky Vox Caster himself survives!
Defence Lines is however the real jewel of this formation. I, like I imagine most IG players, do not have nearly enough Chimeras for all my squads, meaning the majority of my Guard are forced to footslog. This, given the large table sizes in Apoc. means that these Guard are better suited for defending your home objectives while any MEQ's etc. you have strike out at the enemy. Defence lines provides you with a large amount of free fortifications that you can place anywhere in your deployment zone. These fortifications grant a 3+ cover save. Watching the enemy's super heavies and tanks blunt their firepower on IG who suddenly have become MEQ's is fantastic, especially when weathering that first turn salvo. If possible try to use fortifications that will obscure your tanks, we had a predator survive 3 turns of point blank fire from a Baneblade, Leaman Russ & Defiler due to the 3+ save afforded by the defence lines. The frustration this causes the enemy is admittedly pretty enjoyable!
The only really downside of this Formation is its set up time. This formation is best brought out in initial deployment, as a general is going to want to avoid walking the Guard across the table and should rather take full advantage of his fortifications. However attempting to deploy (a minimum) of 75 Guardsmen as well as the Fortifications adds a large chunk to your deployment time, meaning you will often forgo the first turn. However, given the amount of 3+ cover you are afforded this is less of a trade off than it might otherwise be.
'Armoured Fist' Mechanise Company/Platoon
A Tyranid Swarm finds itself advancing under massed sniper fire, only to stumble upon some dangerous terrain.
So there's a brief look at some of the new stuff available to all you Guard players out there. I must stress that, given its price, the new Apocalypse Reload book is well worth it. Do enjoy dropping warheads on your own troops - its the Guard way...