Get a little dry cleaning fluid from your dry cleaner.
Dab on the stain (dry breeches) with a soft white cotton cloth.
If you have any leather on the breeches be sure the dry cleaning fluid does not get on the leather.
I've also had good results just washing them over and over again without letting them dry in between. Sometimes it takes longer than I expect...and I give up, wear them with a shadow of a stain, and a few washes later they're perfect again.