Download only from trusted sites: Most torrent files and crack zip files usually conatins malwares and viruses. Avoid downloading such items.
Disable Guest account: to disable guest access to your system, follow there steps
Click System Properties
Open Accounts > Login options.
Disable "Automatic Login"
Set login window to display Name and Password
Click "Guest Account", disable 'Allow guest to log into this computer'.
Good security software : Virus scanners and security softwares like Clam Xav, Virus Barrier and Kaspersky Anti-virus are made specifically for Mac OS X.
Disable Autofill in Safari : If there are chances of others using your computer, its recommended that you do not use the autofill feature in safari browser.
Open Safari choose, choose "Preferences"from menu.
Goto Autofill tab
Uncheck "Autofill web forms using info from address book card".
Use a strong password: Donot use any easy-to-guess strings like your pet's or Net ID. Combination of two or three words could create a good password.
Stop communications sharing : Communication services like bluetooth keeping in sharing mode in public places can cause the system vulnerable to external access. So when you're not using them , turn it off.
Turn off automatic logins: Unless your're at home, do not enable automatic logins. It may be an annoying job to re-enter the passwords every time the system boots up but it makes your data more secure.
Disable Java: As we already, more than 700,000 computers running Mac infected with Flashback Trojan. This threat affects on systems with outdated java plugin running. Since it's not easier to update java as in windows, easier option is to disable java from within the web browser itself(Safari). you can disable java by going to Applications > Utilities > General tab.