If you're concerned about someone spoofing your account you can choose to prefix your mobile updates and commands with a short PIN (short for "personal identification number"). For security reasons, only you should know your PIN.
To set up your PIN:
If you forget to insert your PIN before entering the text of your message, you will receive a message back stating Your PIN is invalid. If you receive this message just simply resend your SMS with your PIN at the beginning.
If you have forgotten your PIN you can simply log into twitter.com via the web and check in your Mobile Settings. Your most current PIN will be displayed.