0) wifi, make sure it s there for all.
1) power plugs, make sure you have enough
2) tell ppl what to bring, event with several blog posts people kept asking if they had to bring their laptops ( :/ ) ie be very explicit about what they should bring.
3) Add a show and tell at the end. It gives the event a closure and people can show their work, they might not like this initially, but they will a few days later.
4) if you can have food, at least water and coffee. People get very grumpy when they dont have these handy
5) Enjoy it! its great craic :D

There are many blog posts about this and even a book (the game jam survival guide) check them out.

DM me if you have questions, I organized a 120 people game jam at the end of february in Ireland