You know the venue, and when the tournament will be held. You know which army you'll be using, and the missions for each game, but you have yet to design your list. Too many 'competitive' players are inclined to look to the internet and copy what they find, unit/unit equipment/equipment. 'Nob bikers seem to be popular right now, I think i'll run them'...'Vulkan-based melta/flamer spam won tournament x, so i'll bring them to tournament y'. While any given list that you find online may very well be cohesive and flexible, that doesn't necessarily indicate that said list is suited for your particular play style(s).
Each player has their own tendencies in strategies and instinctive reactions on the battlefield. This very often becomes self-evident with the army that any given player takes to a tournament. However, even identical lists can be played in utterly distinctive manners. To illustrate this, lets examine part of a scenario between two different players playing an identical list and two of the (many) possibilities that might ensue. Lets say that both player A and B are running an SM list consisting primarily of tactical squads in rhinos with a meltagun, combi-melta, powerfist and missile launcher in each squad. Player A stylistically enjoys taking advantage of his mobility, and using his units to dictate the shape of the battlefield. So on turn 1, A is more likely to move most of his rhinos 12" forward to get into the face of the enemy. B on the other hand has the general tendency to play marines as a more of a counter-attack army. So B may be more likely to sit all of his rhinos in his deployment zone and fire off his marines' MLs, waiting for the opponent to get closer before counter attacking and employing his meltas/bolters/pfist capabilities. Now, a marine list of the above nature would be more or less flexibly capable of supporting both play styles. However, it is most likely going to be advantageous for these two players to consider emphasizing other units that better support their play styles. Two units of speeders (Mutli-Melta/Heavy Flamer) might be more advantageous to player A than having his nth tactical squad as this would allow him greater mobility, nice flexibility and the ability to block or contest with 24" skimmer movement (ignoring intervening terrain). B on the other hand might want to consider investing in Razorbacks for his tacticals, ranged dreadnoughts or preds to help support his habitual strategies.
"Well shouldn't you play to the situation?" Yes, in every game you play, your tactical decisions should be dictated primarily by your opponent's list, the terrain, the mission and your opponent's play-style/setup. However, their are usually multiple ways that any army can deal with any other army and any given player is more likely to emphasize his/her strategic tendencies than to counter a particular scenario in an 'ideal' way. And how can we determine the 'ideal' response to any in-game situation anyway? Short of working out every given statistical possibility across all X game turns (which would be near impossible in any time frame let alone the usual 2.5 hours you're allotted in tournaments), there is no reliable means to determine the 'best' approach to any in-game situation. So it is helpful to have an army that supports your strategic tendencies, so that you can a. Plan and have a general idea of how your army will handle different scenarios and b. when you're in a timed game and have to make quickly thought out/reactionary decisions, your army generally will be at its strongest when you rely on your instinctual tendencies.
This leads me into the next part of this post, instinctual reactions. While it is important to plan, or work out the statistics of each scenario on the battlefield, when it comes tournament time, time will hinder your strategies and statistics. Even the best players will make 'mistakes' in high pressure competitive situations. And identifying/capitalizing on mistakes is a key to victory in almost any game or sport. Lets examine a particular unit, in a very specific situation: A squad of 20 Termagants (http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat1020009&prodId=prod380003a) standing on an objective on turn 5/6 with a squad of 10 marines bearing down on them at an estimated 13-14" away (requiring a fleet in lieu of shooting to assault). Now, player A (as we describe his nature before) may be more proactive and decide to attempt an assault to try and hold down the marines. While player B may instead decide to sit still and employ the termagants' fleshborer shots at the enemy. Now, if player A is consistently making said in-game decision to assault with his gants instead of shoot, then he may be better off considering hormagaunts, genestealers or warriors as a troops option. Player B on the other hand may be better off equipping his termagants with devourers (allows for 3 shots/model).
So when you're designing your army list(s), be sure to consider how you intend to play the army as well as to observe your general instinctive in-game reactionary decisions. As these should help dictate which units you use and how they're equipped. Playing an army that is designed per your style will help you minimize game-play mistakes and will better capitalize on the tactical decisions that you make.
This is just one aspect of many in list building that needs to be accounted for. However, I will address others in their own articles. Thoughts? Suggestions? Do you require clarifications? Don't hesitate to comment and be forward with your opinion.