Keep it simple and professional. Don't use gimmicks or games or anything like that.
Appoint someone, such as the project manager (or, in Scrum, the ScrumMaster) to solicit feedback from everyone, based on team preferences. This could be via email or written on paper and turned in. Ask for everyone to contribute things they think went well and what can be done to continue, things that didn't go well and how to fix the problems, and general thoughts or comments about the Sprint.
After all the ideas are collected, put them on a tri-fold poster, or make a slide deck, or just handouts. The entire team goes to a conference room, sits around a table, and goes through every point, coming to agreement about how to move forward, fixing problems and keeping the good things going.
Don't forget to timebox the meeting. I can't rememember off the top of my head, but I think that retrospective meetings should be no longer than 2 or 3 hours. Pick an appropriate length of time for your meeting, and stay focused for that time.