My Sadie is a 60+ pound lab/dalmation mix who loves to try to push her way between your legs for a butt scratching. Unfortunatly, I can't offer any suggestions to try and stop it, cause we never have tried to stop her. We did break her of jumping on people for the most part, by making her sit until we came to her to pet her. If she jumped we would just push her away and ignore her until she sat. Maybe if you gently knee you GSD dog away and make her sit before she is petted it would help?