My trainer made me canter with chicken wings so my elbows would be loose, but I never did it at the trot before. So maybe I should try that when I go to free ride?
What I do is think when I rise, to stretch my hands down to my horse's neck, when I fall (sit back down) then I bring my arms in close.
So I think
"Reach, take, reach, take"
It helps to stretch before you ride, and I think once you learn not to push your feet down when you aren't in 2-point, as in bracing against the stirrup, then it'll turn out a lot better :)