Document the incident(s) that form the basis of your suit against the police department. In most cases where individuals sue the authorities, it is because the police have trampled on the individual's rights in some fashion, whether it be during a search and seizure or during a person's stay at the jail. Make careful notes of the events.
Conduct some research online or in back issues of your local newspaper to see if the police department has been sued for similar reasons before. This will help you provide your attorney with corroborating evidence of prior police corruption.
Hire a top-drawer attorney to handle your case. You would be well-advised to hire an attorney from out-of-town without local affiliations to the police department. This will ensure that your case will be looked at with fresh eyes and managed accordingly.
Stand up for your rights. Be prepared to do whatever is necessary to get the police behaviors that were wrong out in the open. This can include going to the press and discussing the situation with a reporter to help get the public involved in and become more aware of the issues at hand.
Stay the course. A trial can be long and arduous, so if you decide to sue a police department, you'd best prepare for a long haul.