Quite simply think about what the enemy is going to be bringing and see if you can counter it wether that is lots of 2+ saves, lots of bodies, vehicles high toughness etc. while trying to balance your own objective skills. A lot of the time you will have to skimp on some areas but as you play more games you will realse what is good or bad about what you have used.
I'm not a tournament player like
but as far as I'm aware, this isn't the way to win a tournament - you win not by bringing counters to everything the enemy can bring (impossible due to the diversity in 7th anyway, in my opinion), but by bringing one thing to the extent of all else. Bring an army of nothing but Venoms with 3-man units of Wracks inside. Bring an army of Land Raiders with minimal Black Templar units inside. Bring an army of Knights. Do one thing really, really well and force the enemy to react to you.
Leastways, that's the way I would have thought it worked (and why I thought the CronAir and Leafblower and Razorwolf and Serpent Spam and Tritide lists worked - the only difference being there that the unit being saturated was already a very efficient one, therefore making the saturation stronger than, say, saturating Land Raiders which is far less common at tournaments).