1. Remember to sign out. Especially when using a public computer,
2. Be careful about sending certain sensitive information via email. Once you send an email, you're no longer in control of the information it contains. The recipients, if they so choose, could forward the email or post its contents in a public place.
3. Always check the last login details when you login again at the bottom of the gmail
4. Be wary of unexpected attachments. although gmail scan all the attachments but no system is foolproof
5. Make sure your account recovery information is up-to-date. Hacking of your account can be done from here
Official Gmail Blog: Gmail account security tips