Wear a technical riding shirt (with a long, grippy tail). Instead of tucking it into the pants top, pull it down so it lies flat against your skin, then pull your breeches up over it. The pants top disappears under the pressure of your lower back against the cantle.
A Rider Needs No Pants: The Bold Evolution of Equestrian and Cycling Apparel a rider needs no pants top
If you are participating in a "No Pants" ride (like the annual subway or tube ride events), the "top" refers to the clothing worn above the waist to contrast with the lack of trousers. Wear a technical riding shirt (with a long, grippy tail)
If you’re looking to embrace this trend, the "top" is the most important piece of the puzzle. Look for: a rider needs no pants top