The best way to preserve evidence is through photos of injuries, medical certificates, or anonymous forensic evidence collection (ASS). Anonymous forensic evidence collection is a medical examination that can be conducted confidentially and free of charge after a sexual offense at the victim's request. The evidence of the crime is documented and preserved. Important: You do not have to file a report for this!
In some places in Germany, so-called violence protection clinics offer confidential forensic assessments and documentation of cases of intimate partner violence and sexual violence. This documentation is independent of a police report and is usually free of charge. It can be retained for years and used in both criminal proceedings and in violence protection or family court proceedings.