Join Date: Sep 2011
Location: Germany- but not German =D
Firstly, dentist, saddle, and back checked. Eliminate ANY pain.
Secondly, is it tiredness? Is she working in an outline for a long period of time? If she is, she may need a break.
If you've ruled out all those, and its out of a mini temper tantrum, send her forwards, don't give her anything to fight against. I ride English, and I don't know how different Western is, but if my mare shakes because she's trying to escape something, she's either unsure, or being a baggage. If she carries on, I'll send her forwards, give and take with my outside rein so she has nothing to pull against and let her figure it out on her own.
Teeth check, back check, saddle check. Very important :)