Applying a cold compress can be a good start. Just like in real life, if someone in a story has a sore behind from spanking, a cold cloth or even a bag of ice wrapped in a towel can help reduce the swelling and numb the pain a bit. It's a simple and immediate remedy.
There was a woman who was having trouble sleeping due to anxiety. She decided to try a relaxation massage. The massage parlor had a warm and inviting atmosphere. The masseuse started with a scalp massage which was very soothing. Then she worked on the neck and shoulders. As the massage progressed, the woman felt her anxiety melting away. By the end of the session, she was so relaxed that she went home and had the best sleep she'd had in weeks.