Just my opinion to add to what others have given you here.
The single most important step to using masteries correctly is having a set of keybinds that are comfortable and well organized for how you think. I keep all 9 masteries on a 3x3 block of my bars and a 3x3 block of my keys that match (E-R-T, F-G-H, C-V-B) with base builders on 1-2-3 and other gambit stuff dual bound to mouse buttons and shift+numbers in case I am more busy with one hand or the other.
Tackling the learning of masteries is the toughest part, this is how I started. Take a few of your most-used gambits one at a time and figure out what masteries can be used to build those gambits either partially or fully. IE You can build power attack with [Sp-Sh][Fi], [Sp][Sh-Fi], or even faster by doing [Sp-Sh][Fi-Fi]. Now just go out and grind some Virtues while practicing building those gambits with masteries until you feel comfortable enough to use them while leveling more. Just take it slow and incorporate more and more masteries into your rotations as you get comfortable.
The final goal with masteries is to be able not only plan out your masteries on a short term basis (rotation by rotation), but be able to look at your remaining masteries and know what gambits you can build or think of a gambit and know which masteries build it.
Also, check out the buffbars and combat analysis plugins if you haven't. I find them very useful on a warden.
Note on this screenshot: I use a G9x and thus have only 3 mouse buttons to work with. Almost everything is keybound, instead. I don't like Ctrl modifiers, so I ONLY use it for stance dancing.