I actually think this may be the most important part of the game, as far as winning goes. There is a lot of luck in the game, just look how frequently you roll dice. Selecting your forces has perhaps the most influence on determining a winner in 40k. Although people often build hypothetical lists, I personally only field the models I have painted. View the full compliment of the Diseased Sons.
Again all of this advice is out of date as I have not played the current edition and barely played the previous edition.
The first decisions you have to make is the mandatory choices an HQ and two Troop choices. Obviously if you found this site you know that, however it really is the basis of army selection. I'm still searching for the optimal leader, but I've extremely simplified troop selection, basically I take Plague Marines.
My most recent leader which I converted and unleashed on the unsuspecting peons of Vancouver will be Nurgling infested. Manreaper should prove a better option assuming you want to get into H2H. I wish to try a more shooty army and I will lead it with a Lord with Kai Gun to be converted sometime in the future. The new codex has so many options I would have liked to play a bunch before I come up with my definitive Vancouver GT list but I got the basics down even before the Codex came out.
My army is always based around Plague Marines usually in 7 man squads. I also try to take Plaguebearers and Nurglings though the latter has very little practical use.
I always try to take all allowable special weapons. Though as to the best special weapon, my thinking has changed over the years. Flamers used to be more useful when there were extreme 'to hit modifiers' such as on Harlequin jet bikes in second edition. It seems Plasma Guns are the most popular in 3rd edition but I tend to lean towards Melta Guns. I often forget that Plasma Pistols are a valid special weapon for Plague Marines and haven't painted too many models to take advantages of this rule.
Previously Plague Marines had to have Bolters, then they could be made into straight up assault troops I busily painted and converted Bolt Pistol and Knife models. Now they've changed the rules again to favour the bolter in my opinion. All I can say is "Good Riddance Plague Knifes, Hello Variety." I like to buy Rhinos for the assault squads. I'm less a supporter of Blight Grenades. Now that they cost extra I'll defiently do without.
The free Champion is a recent development, you can not complain about free. I think champions should have minimal gear. Why spend piles of points on a one wound model? My thinking has changed slightly with the new codex as equipment is cheaper for Aspiring Champs so I'm arming them to be more combat effective now. I always use a model that has at least one piece of wargear to set them apart from their fellow squad members. Usually something to help in H2H, an effective purchase can be a Plasma Pistol though the Powerglove has new allure and who doesn't like a Power Weapon. The new Codex just gives you so much.
I think the ratio between assault, tactical, and heavy support is important. My army has always been painted with this ratio in mind, with a definite leaning towards assault. For the next GT with the new 'dex I want to be shootier. So three Heavy Support and a big gun line with a sprinkling of assault elements is my new plan. I've dubbed this upcoming army Nurgle 3.0. I may limit myself to two Heavy Support choices in the hopes of increasing army comp. scores and instead take terminators with reapers.
For the assault oriented squads I usually bring a flamer. The Melta is also an excellent choice for assault squads. The lack of range is not an issue if you use rhinos. In a pinch I may not bring a special weapon. The new Codex gives back to Nurgle the ability to infiltrate special weapons and assault squads. I loved infiltration in the second edition. I also try to summon my demons to support my assault squads. In a pinch you can summon the demons at the back of your army to protect your support troops, don't forget this.
The randomness of the demons arrival means they can not be relied upon to kill something. I prefer to rely on them to overkill something. I charge with or without them and rely on T5 and Power Armor to last until they arrive. I even regularly use them as a screen. They are usually cheaper and they have an unmodifiable save so they are frequently called upon to take Lascannons for the team.
Nurglings will not kill anything, repeat it seven times, then field them anyway. They can't really be used as a screen, especially as clarified in the new 'dex. But they can hold up assault troops. I usually field them either along with my central assault force or to protect a Heavy Support squad or vehicle from being assaulted.
As for a leader I try not to spend too many points. I usually equip them for H2H and take wargear to help them get across the board in one piece. I've taken to using a deamon lord with wings towards this end. But with the new Codex and having faced them in Calgary I'm all about a Landraider to drive my lord about in style. This is nice thematically but not very effective in game terms, it concentrates everything into one expensive target. As I like my Lords cheap I've never fielded one with Psychic powers, but a sorcerer in a retinue seems like a great idea. The new secondary chaos hero will be of great use in larger games, in smaller ones I think one HQ choice is enough.
The Achilles heal of Nurgle is long ranged fire power. This is where you have to sacrifice. I've slowly built up a large collection of vehicles. Predators are a good value, though for sheer intimiatation you cannot beat a Landraider. Dreads have versatility and are not vulnerable to H2H like most vehicles. Defilers are simply cool, and barrage weapons fits my image of Nurgle.
I plan to add additional vehicles to my collection prior to assembling another GT army. I want to expirment with a sponsonless predator as well as a third dread using the nifty new Forgeworld model. I also have maniacal schemes for multiple Defilers now that model has been released. Don't get me started on the Warhound Titan. I just don't have the much money right now...
Although I've tried and then dismissed Plague marine Havoc squads, I've been reconsidering them of late. I think in a rhino they could be useful. I've even schemed of a three plague marine havoc squad army supported by scads of Plaguebearers as troop choices.
So in conclusion I hope you noticed I made no mention of my opponent. I don't overly consider them. I field the models I have, leaning towards the ones that look cooler (I got away from that a bit for GT Calgary, I intend to field an even cooler army for GT Vancouver), which is usually the figures I've most recently painted. I try to strike a balance between pure assault troops (Plaguebearers), mostly assault troops (Plague Marines with bolt pistols and thus an extra H2H attacks), tactical squads (traditional Plague Marines and even the Havocs to a lesser extent), and long range support (Dread and tanks). My lord is for assaulting though I continue to rethink this. George and the George Jr's do nothing, but they have potential to be annoying. I mean if you've had your models killed by vomit or by Nurglings you don't forget that. You're scared for life.
Though Nathan and others are always enamoured with the Possessed Marines. I've manage to live without them, but Chosen they are the best troops in the Chaos Codex IMHO. I'm taking a retinue of them to Vancouver GT. I'm reconsidering Terminators outside of retinues as well. In my quest for a shootier army, I noticed every third Chosen could have a heavy weapon, for Nurgle this can only be taken advantage of using Terminators. This is especially clarified in the then Chaos Marine FAQ. So I intend to continue to experiment with terminators. I've been rethinking the Possessed for visuals alone I may make some, mine will most likely have deamonic flight.