To be fair, opening your hips isn't always something that you can just do. It's a flexibility issue (if you google it, you'll find approximately one billion yoga exercises for "hip opening!") that needs to be developed over time.
I can't think of a good way to describe the biomechanical opening of hips, so here-- sit on your chair. Then, tilt your pelvis up while opening your thighs and letting them drape down. You'll probably feel some stretch! That's what I'm talking about. It's when you can sit on your seatbones but still allow your legs to "drape" down alongside the barrel of the horse. It's tough! I will admit... for a few months, I did a lot of yoga and (this is embarrassing!) put my velcro knee/thigh blocks in a very aggressive position on my saddle just to force my hip angle open, because I was so tight! I'm not recommending it, but man did it feel good to stretch that out while I was riding. I find that the flexibility of your hip angle is very important when it comes to sitting the trot, since the motion requires your hips to open (close a little) open (close a little) open with every step.
I hope that helps a little bit! Think of it as more of a long-term goal rather than something you can fix in a day. :) It's tricky to keep open hips considering how sedentary our lifestyles can be!
ETA... if you look at your most recent EQ picture, imagine if your leg stayed in that exact position, but your pelvis tilted up so that you are sitting more on your seatbones. *That* is opening the hip angle!
Last edited by existentialpony; 11-04-2013 at 02:26 AM.