I have found the best thing after riding our horse for sore muscles etc, is to continue to be active for at least fifteen minutes. Take extra time cooling down your horse as your putting him or her away. Walk, keep moving. Your muscles will become more relaxed and tone and appreciate you more.
Believe it or not - going horseback riding again - the next day. LOL Really. It's probably not realistic though. Stretch a lot, take a hot bath and go to bed early (get lots of sleep).
Take a hot bath with Epson Salt added. This helps to relieve the soreness in muscles.
You can try some valerian, an herb that relaxes muscles. Or, try a calcium/msgnesium supplement. It'll relax your muscles, also.