Yesterday I was doing a demo for one of our customers in their office.
Nowadays Wi-Fi is a dime a dozen and usually companies have dedicated Wi-Fi networks for their Guests.

But what if they don't? Or the client is not technical enough to help You, or their IT staff is not in place?
Are You going to fail the demo or blame the customer and loose Your face?

Her is my piece of advice: To be prepared I always have the following things in my suitcase: A smartphone with an ability to launch a Personal Hotspot ( the way to share your cellular data connection via Wi-Fi, actually Apple can explain it better http://support.apple.com/en-us/HT4517).
All modern smartphones have this feature now.

My trick here is that everything is setup upfront. So I don't spend valuable customer's time doing some acrobatics setting up the connection. I just launch it and it works out of the box.

As a last resort my suitcase always has thunderbolt-to-gigabit-ethernet-adapter (http://store.apple.com/us/product/MD463ZM/A/thunderbolt-to-gigabit-ethernet-adapter) for my MacBook and a small piece of RJ 45 cable.