i feel your pain! i myself have a bit larger bust size and found the sitting trot to be rather uncomfortable in that region when i don't have the bust support i need.
what i do is double up on bras, underneath goes the underwire bra that supports me just fine when not doing strenuous activities. over top of that goes a fairly snug fitting sports bra that doesn't have any shaping, etc. basically, i strap those gals in! lol. while two bras may not be the solution for everyone, it works for me.
ETA: SlideStop posted exactly my same solution while i was typing, lol.