Positioning is based on spotting advantages. The driving question is Where do things need to be before the chaos begins?
[Stelek's 5th Edition Daemon deployment defense would be a great example of having a plan and adopting it to the circumstances. He has the plan already, but it is because he is playing Daemons that he can use it. He then makes note of the terrain to provide traps for deep striking daemons and keeping them away from his critical units.]
To plan, you start by identifying critical logical dependencies between necessary actions and time constraints. You plan by making commitments around those things.
To position, you start by identifying critical likely patterns of information flow after tempo epoch shifts. You position by identifying likely islands of calm in the forthcoming information churn. Think about surveying a flood plain that you know will start filling up with swirling waters and howling winds soon. Where are the relatively safe places? What can you do to prepare in those places?
[A lot of Internet opinion is that your battle fortunes are dependent upon your deployment. For Blood Angels players, we need to think about also our reserves and redeploying because of our greater speed than other marine armies. What look like nice places to drop in an Assault Squad or drive your Vindicator 12" onto the board and fire that large blast template. This also means delivering those Assault Terminators into the path of the enemy or force them into a firing lane.]
Preparing for a melee though, is more like training for a sport by raising general fitness levels as high as you can.