Very practical things
Sign, sign, everywhere a sign. You can't have too many signs for wifi, Twitter tags, arrows pointing to the bathrooms and exits, put the agenda in a very public place, etc..
You can't have enough mobile whiteboards, flipcharts, and other public writing surfaces.
5-minute breaks between sessions. People need time to get from one room to another (or find out what room they're headed to next).
Work in time for longer breaks. We tend to want to pack a day as full as possible. Don't. People will take breaks whether you schedule them or not.
Clocks in the rooms. Also having someone go around and announce how much time is left in each session is good.
Cloakroom. People tend to put bags and coats on chairs, which inhibits their use by other peopl