After having many live demos go awry, I attempt to avoid live demos as much as possible. That being said, a few thoughts, in no particular order.

I prefer to rely on screenshots and animation which can typically illustrate my product as well, if not better than an actual demo.

Screenshots and animation allow me to craft a narrative that focuses on the products key value proposition.

A live demo often results in an audience member asking a question, then diving deep down into some sort of non-critical feature. A demo based on screenshots with animation will allow you to stay focused on our key narratives and primary value proposition.

If you must give a live demo, you could use a wireless hotspot instead of relying on somebody else's network. Another option would be to run the demo on a remote machine, and show it using a screen sharing tool (such as join.me, WebEx, etc.), so that you're only relying on the firewall to allow HTTP traffic. (Though, that might have a lot of latency.)