sounds familiar... try buying the biggest pair of granny knickers you can
get - works for me!! (less so for my husband...)
Alternatively and slightly less ugly - Mountain Horse does riding knickers
which are quite good & stay in place
Reminds me of an exchange we had here a while ago where someone said she
did a 50 miler in a string and felt she was flossing her butt off haha!! That
image has stayed with me ever since - ough