A quick #FOSDEM guide from someone who's been attending since it was still called OSDEM:

1. Be mindful of other people. This is an extremely busy conference.
2. If a talk is in progress and you want to enter the room: don't. Wait until the next talk.
3. If you didn't read 2, try to enter as quietly as humanly possible, and ideally even quieter. Many of the rooms have terrible acoustics and you will annoy hundreds of people.
4. The hallway track is very busy. Take it outside when possible.