Whether you should file a police report without pressing charges depends entirely on the circumstances of the event and jurisdiction. Some matters, deemed criminal, are automatically investigated by the police. For example, if a person threatens you with a weapon, the police will investigate out of concern for public safety. You, on the other hand, do not have to press charges in a civil court. Other matters may be investigated only at the request of the person making the complaint, such as if someone trespasses on your property. A person may file a police report so there will be a record of the incident without choosing to insist on a police investigation.
Take detailed notes of the incident. Note all key details, including time, place and description of persons involved. Also note contact information of any witnesses. Take the notes immediately, if possible, and encourage witnesses to write statements of what transpired. The more evidence collected immediately and the more detailed the records kept, the stronger the case will be if the victim decides later to press charges. Preserve the crime scene and any physical evidence, if applicable.
Visit or call the police immediately. File a detailed report with the investigating officer and supply him with all evidence.
Tell the police that you do not wish to press charges. Filing a report gives the victim a chance to press charges in the future (within a certain time) and lets the police know about someone who could potentially be a problem for the community. Victims of sexual assault may be too traumatised to press charges immediately, but should file a police report to protect others and preserve physical evidence.
Retain legal counsel if you want to pursue the matter in a civil but not a criminal court. Civil court works to give the victim reparations for the crime without charging the accused with the crime. For example, if a friend steals your TV and refuses to give it back, you may not want to see him go to jail, but may want to get your TV back.