What I've done over the years is pay attention when I'm grouped with a good player.
For example, I found myself grouped with an Illusionist who was parsing consistently high. I made sure I saved his parses in ACT, then I studied them, then I re-ordered my Priority and CA tabs accordingly. Then I respecced my AA to match his. Then I swapped out my adorns to adorn like he did. Then I re-forged all my gear to re-forge into the stats he was re-forging into and out of.
It took time, of course, but my raid Illy's went from parsing 200k in raid to 400-600k in raid (all in EM gear).
Finally, independent of this, you have to make sure you're buffing your groups correctly. That's not so easy to learn, but if you master it, your groups' DPS can really go sky high.
Good luck!