I second the recommendation to turn off everything in your Sharing preference. In addition, you might want to turn off Bluetooth if you aren't using it, or at least turn off Discoverable there on the Bluetooth menu.
Just know that anything that isn't encrypted can be sniffed by anyone else on the same network because it's sent as plain text. That means email, HTTP (web pages you visit), iChat, Twitter, blog entries you upload, iPhoto & iTunes libraries you have shared, FTP, telnet, etc.
You can encrypt some things. If you can connect to your bank etc. with an HTTPS address (lock icon), that's safe. If you turn on SSL for your email, that's safe.
But for everything else, the only practical protection is to use a VPN that encrypts all your traffic from your laptop all the way out to the VPN server on the Internet. No one else can see what's in there. These companies provide such services; I am sure there are more and I do not officially endorse any although they do work.
Hot Spot VPN
Witopia
Public VPN