Lock down your Mac: Step 2
Next, go to the Firewall tab and turn the Firewall on. Depending on what software you're running in the background, you'll likely get immediate prompts to allow or deny incoming connections. I recommend denying all of them, finding out what's not working, and then opening only the necessary ports.
FireVault, another option, encrypts files. It adds a layer of protection, especially for mobile users, but in my experience it doesn't play well with Dropbox.