Gaining extra focus by priming abilities
This is a guide for something I'm sure some hunters know, and most hunters DO, however, I'm also relatively certain most hunters don't realize that they're doing it.
Why this works
When you're in the middle of casting a spell and you try to cast another spell within about .5 seconds from the end of the original cast, it primes the second ability to use automatically as the first ability finishes. This effectively allows a hunter to shoot Steady Shot or Cobra Shot at the exact same time as a second ability that doesn't have a cast time.
Cobra and Steady Shot both deliver their 9 focus as the shot actually hits an enemy, unlike shots with focus costs, which spend the focus AS the shot is made.
What this means is that by priming an ability like serpent sting at the beginning of a fight by casting Cobra Shot first, your 100 focus will go down to 75 before recovering 9 despite having shot Cobra Shot first.
Video documenting how this looks
Method
When you want to employ this tactic, simply cast Cobra Shot or Steady Shot, and when the cast bar is almost full, as pictured below, start to cast any other shot.

What exactly does this do for me?
This allows you to start a fight using Cobra Shot with any focus-cost shot, starting the fight with a boost of damage and 9 focus MORE than you would otherwise.
I was struggling with whether this was an exploit or not, but I don't think there was anything against any intended Blizzard mechanic. Just a clever use of what we're given.
Hope you enjoy!