Left to their own devices, people will game the system to avoid confronting the unpleasant.
Vary the way you to the postmortems. By definition, they're supposed to be about lessons learned, so if you repeat the same format, you tend to find the same lessons, which isn't productive.
...ask each group to list the top five things they would do again and the top five things they wouldn't do.
Employ lots of data in the review.
Most of our processes involve activities and deliverables that can be quantified.
Data can show things in a neutral way, which can stimulate discussion and challenge assumptions arising from personal impressions.
On a side note.... Don't EVER, EVER, EVER Google the phrase "Postmortem" in an effort to find a nice photo to go along with a blog post. EVER.