Cosequin is wonderful and will help with the arthritis associated with luxating patellas, but it won't fix them or prevent them from getting worse. Keeping her from jumping on and off furniture is the best way to keep them from getting worse. Dogs with luxating patellas are also prone to ACL tears so it's very important that they don't jump.
Did your vet grade the luxation? Grade 1 doesn't need surgery, Grade 2 probably won't, but grades 3 & 4 will need surgery.
I get Lady's Cosequin from Entirely Pets:
Cosequin Regular Strength (90 capsules)