One thing we took away from Freebootaz (yes I'm still going on about it) was that no matter who we faced we were confident about our list. We knew exactly what each unit's role was and their strengths/weaknesses. That doesn't mean we never got it wrong but at least we had an idea in our heads.
Essentially with a good list the hard work is all done for you. You've already stacked the odds in your favour and it's down to you to react to the changing conditions of the game. Ultimately a good list can be the difference between winning and losing. So what makes a good list then?
1. Play to your armies strengths
We were using BA and Wolves. The Blood Angels are at their deadliest when in combat so we took their most deadly combat units. No we didn't bring Death Company but Rage makes them an awkward choice in objective games. The Space Wolves are a little more difficult to define. You may quickly dismiss them as another combat chapter but in actual fact they have some of the best shooters in the game in the form of the dependable Long Fangs. They also have arguably the most flexible basic troops in the game with the "swiss army knife" Grey Hunters. There's no point taking a devastator squad for the Blood Angels if it means sacrificing an assault unit but for the Space Wolves a mixture is what it's all about. You only have to look at our special characters to know this. Logan's High King ability gives Fearless, Tank Hunters, Relentless or Preferred Enemy. That says to me that you can stick him with shooters or fighters and he'll be happy.
Everyone wants to have an army that's good at everything but that simply isn't the way GW designed them. As good as Long Fangs are they aren't better at hitting troops than Imperial Guard nor are they better at popping tanks than the Tau.
2. For Every Unit A Plan (or Two)
If you're planning to do well in a tournament and not just playing a mate for fun then don't include anything in your army that you "just like the model". I do this all the time against Matt and it's probably why I lose so much. If you have a rough idea of what army you're going to face think about the units they're likely to bring and plan your army as the rock to their scissors. No one likes to see the perfect counter to their plans sitting opposite them on the table.
Think about what every unit in your army is going to do. For example, if I put in a squad of terminators I know I'm going to pile them into a high value target as soon as I can either by Land Raider or Drop Pod and make the most of their large number of power weapon attacks. If there isn't a clear target for them then I'll think about having them on my objective and make them as hard to remove as possible. The key is to be flexible. If you bring a strong anti tank unit and your opponent fields his whole army on foot then think about what they will target instead. Instant Death is your friend and remember AP1 can be just as deadly to troops.
3. Draw Their Attention
In every army have something that's going to take your opponents' eyes off the prize. We used Mephiston to this effect. Your opponent sees this deadly model and knows that he's a bitch to kill so he piles everything he has into bringing him down before combat. In practice he soaks up most of it and probably still gets in to do some damage. Even if he dies he's distracted your opponent long enough to get your transports to his lines unhindered. It may seem like your opponent will be smarter than this but we've all done it. You want to deny him the glory of getting that guy into combat or that anti-tank squad ever bagging your land raider, you turn your attention to destroying him/them above anything else and before you know it, the rest of the army that you weren't as worried about has bitten you in the ass.
If your army is bland then your opponent will likely just shoot the closest thing. This can be used to your advantage also but it's nowhere near as effective. This doesn't mean that every opponent will fall for it but I guarantee you'll be surprised how many take the bait.
4. Keep it Simple Stupid
If you don't know your army like the back of your hand you've already lost. Nothing makes this more difficult than an excessive number of upgrades for each squad that you simply forget you have! Likewise making each squad unique with different weapons etc means you're almost certain to end up with the wrong squad in the wrong place at the wrong time.
Only add units/characters with special rules you're going to remember. We forgot to use Red Thirst half of the time and it could make the difference. Likewise I deliberately didn't take any Wolf Standards because I always forget to declare their use in the shooting phase and it isn't until I've rolled a few 1s that I remember!
If all else fails don't be afraid to scrawl the words "Red Thirst" down your arm in marker pen so that you don't take that risk. Part of me thinks we should've done it but don't forget to wash it off or your wife/friends/colleagues will wonder if you've lost the plot!
5. Spent Those Points Wisely
Start off with vanilla units and build up from there. It's very tempting to give them all upgrades but it's easier to budget for those upgrades once you've fleshed your army out. There's going to be things you'll always take like meltaguns for your hunters but any other "luxuries" like MotW or Standards should be added on last.
For any upgrade you apply think about what else you could buy for the points. Power weapons, plasma pistols and MotW are all 15pts which is the same as another Grey Hunter. Will that upgrade perform better than another guy on the ground? Remember that points quickly add up. Wolftooth Necklaces may only be 10pts but give 3 characters one and thats two Grey Hunters.
Weigh up the benefit of the upgrade against it's points cost. For example a plasma pistol may seem like a good way to spend some leftover points but for the one time it's of some use in the game will it be worth it?
Do your best to spend every single damn point available. If you come out with a 1494pt list then find something to spend 5pts on! That or rearrange things so you've got 15pts to spend instead.
Finally, ignore mathammer posts on your favourite 40K forum. There, I said it! They are universally calculating the odds for what is already a very specific situation. For example, the AC vs Lascannon debate. The assault cannon looks great after the mathammer hits as it seems to outperform our beloved lascannon against nearly all armour values. Not to mention it's more useful against troops should you have no vehicles to hit. Before you rush out to convert all your razorbacks though think about the reality. Are those razorbacks really going to be firing at much with 24" range? If they do how long will they survive with AV10-11 at that range? Does it save you any points to lose your S9? Whats that AC going to do when you require instant death on a target?
Use your common sense. Mathammer has it's place for making you think outside the box and try things you might not have done before but don't assume because it's "statistically more efficient" that it's worth the work to convert your models. Most Mathammer is against MEQ too. Whilst there's a lot of it out there what are you going to do when there's a horde of tyranids heading towards you?
As someone with a scientific background I realise that any mathematical model makes certain assumptions. Things like the unit will be at full strength, they won't be in cover, they won't be affected by terrain, their target won't have any special rules etc etc. These ideal mathematical conditions don't exist in the heat of the battle unless you plan on taking everything into account and getting your calculator out before rolling every single dice!
There's a lot to think about when writing a list then? Why not try some out that you find on other peoples' blogs or on forums but remember they'll suit their owner's style of play and not yours. The best way to pick a list is to play test it. Yes, you can tell how well unit A will perform against unit B on paper but until you're in the thick of the battle you'll never really be sure. I'm far from perfecting my 2K All-Comers list but I'm getting there slowly!