How to protect my Google Account?
Instead of entering a password every time you sync or access your mails - which will also be more insecure, because you enter the password often and in public - think about a good screen lock (PIN or Password) and a remote wipe solution.

Changing your Google account password is always a good idea in case of a stolen Android device. According to Oleg Ostroumov comment, the device will stay logged in for ~hour and then request a password.