Use fingers, butt plugs, or dildos with plenty of lube to slowly relax the sphincter muscles before intercourse.
Start in a controlled environment, like an arena, with a calm horse you trust. Safer Barebacking Pamphlet Feb 2023 barebacking
Avoid "gripping" the horse with your knees, which can make you bounce. Instead, let your weight sink deep into your seat and follow the horse's motion. Use fingers, butt plugs, or dildos with plenty