Hey there, SandDragon! Interesting question :)
Like Keromatsu correctly said, there is no professional way of selecting natures, nor is there a system to do so. It all depends on your instinct and your views on the given Pokemon. For example:
Azumarill @ Choice Band
Ability: Huge Power
EVs: 172 HP / 252 Atk / 84 Spe
- Play Rough
- Aqua Jet
Take a look at this set. Notice that I've used the Adamant Nature. Can you guess why? Well, I used it because everything I gave to Azumarill helps it attack physically. The Adamant Nature increases Attack (which is useful), and decreases Sp.Atk (which we don't need anyway).
Therefore, professional players look at a) the Pokemon's set and b) the Pokemon's base stat, before choosing the nature.