Prevent, Soothe Soreness
Always warm your horse up adequately before beginning a work session, then cool him down afterward before putting him away.
Increase both the duration and the intensity of work incrementally over weeks.
Provide rest periods as needed during work and between riding sessions to avoid overtraining your horse.
Consider a bracing liniment to soothe any post-workout soreness or to loosen especially stiff areas before riding.