Heres one to help with keeping bent arms and thumbs up:
Let your arms hang down by your side , like dead meat. Now, roll the lower arm so that the palm faces outward as far as you can without straining. Then, without changing the fall of the upper arm, or the position of your shoulder one iota, bring your lower arm up in front of you as if to hold imaginary reins.
You should still feel a slight pull across the front of your chest, under your collar bone and along the front of your shoulder socket. Enhance that a tiny bit by dropping your elbow down toward your hip. I bet you can drop it at least half an inch.
Dont' be stiff, but try to remember this position. Pick up the reins and walk on. Imagine that you have a million dollar bill under your armpit, sticking out toward the back. You need to keep that upper arm connected (not rigid) to your rib cage, or you'll lose that money!
Now here's a kicker! When you want to ask your horse to soften to the inside rein, instead of pulling or flaping with your wrist (not saying you do this, but a lot of riders do, including me ) , you literally squeeze that underarm place with a rythic pulse that mimics the very soft rythmic pulse in your finger. This will really encourage your horse to soften while at the same time keep you connect to YOUR core.. try this excersize in a dinner chair as you pretend to soften your horse for an inside bend, change the bend, ask for a half halt.
Keep your head hanging froma puppet string all through this, and BREATHE.
Try this and tell me what it feels like.
And ask instructor for lunge line lessons, NO REINS.
Hope this helps,