Instead of worrying about elbows, worry about keeping soft, steady constant hands following the mouth of the horse. I find that focusing on one area usually stiffens a person up
Sometimes I have to "roll my shoulders" to relax them and then everything else follows
Well it's always important to have a nice form.... but to always relax! Once you relax you and the horse will fall into 1 motion together! So just relax.... usually some stiffness comes from nervousness or just habit! My tip for you is to relax everything but keep your form!
Relax your shoulders. Everything comes from them :)
I like to press my shoulders together, it not only adjusts me but gives you good form!