Below is my understanding of how focus works (from a Hunters Perspective and level 85). Please let me know what I get wrong:
Maximum Focus: 9.
There are two MAIN ways to quickly add focus:
1) Out of combat - Improved focus. This will get you to Max Focus in about 3 seconds if you are at zero.
2) In combat - Intent Concentration. Adds 9 to Focus (MAX) but has a 3 minute cool down)
3) In combat - Stance Precision. Adds one focus every 5 seconds.
Out of Combat Focus Loss.
Focus slowly drops out of combat as a function of time, BUT running will make it drop at about the rate of 1 Focus point every 2 seconds.
In combat the major drivers of focus are skills used during combat. Some add focus, some use focus.
+1 Improved Quick shot. (but only a 30% chance to apply??)
+1 Barbed Arrow
+1 Bards Arrow
+1 Improved Split Shot
+1 Improved Distracting Shot
+2 Improved Swift Bow
-6 Improved Merciful Shot
-6 Rain of Thorns
-1 Hunters Art
-3 Rain of Arrows
-3 Blood Arrow
-3 Improved Penetrating Shot
-6 Fleetness ... but focus loss for movement for 30 seconds.
Special: Improved Scourging Blow makes the next Barbed Arrow use no Focus but times out if not used within 9 seconds.
Why Focus is Important
Because if you run out of focus then you can't use any skill that removes focus.
Based on the above there seems to be two main tactics for focus.
1) Stay at range and use a sequence that means you are adding as much as you are using for Focus.
2) Get into melee range so that you can use Blindside and perhaps improved scourging blow to help focus use.
Let me know what is wrong with the above. Please note I deliberately did not talk about induction times and power cost in the above. I want to make sure I understand Focus first LOL.
Feel free to tell me want needs to be fixed.