Process every employee complaint without commenting on its merits or on the potential consequences of making the complaint. Remind managers to do the same.
Never make snide comments such as, "This doesn't seem like a big deal," or "A harassment claim could make it harder to work with your boss." Such comments could be viewed as prejudgment of a complaint, or evidence that the investigation won't be neutral. Employees may see your comments as an indictment of their complaints and evidence that they'll be punished for rocking the boat.
Recent case: Jordan Norton was a power plant technician and an active Mormon. After a supervisor criticized Norton in hisfor an inability to get along with others, Norton fired off an internal complaint about harassment and sex discrimination. He told an HR person that he was tired of "hearing the Lord's name used in vain," and that his co-workers forced him to look at pornography. He also claimed the company maintained a sexually hostile work environment, citing boorish language by co-workers.
The supervisor warned Norton that his complaint would only stir the pot further, and that co-workers would take a dim view if told to clean up their language. From that comment, Norton concluded that his supervisor didn't think much of his complaint. After the company fired Norton because his performance didn't improve, he sued, alleging retaliation. The impetus of Norton's lawsuit was that his supervisor treated his complaint lightly. (Norton v. Firstenergy Corp., No 05-JE-5, Ohio Ct. Appeals, 2006)
Bottom line: Remind supervisors to treat each complaint with a straight face, and let the process determine whether the complaint is valid. Explain that any retaliation against complainers is illegal, even if the initial complaint has no merit.