Unauthorized access to your account is prevented by your username and password. Set a unique password or passphrase that you can easily remember, but never write it down or disclose it with anybody, not even William Hills. Use a mixture of numbers and letters rather than a vocabulary word.
Your security question responses, among other account details, are used to verify your identity when contacting Customer Service. As a result, the answer to your security question must be tough to predict and not anything that can be found on social networking sites such as Facebook. Why not set your security question to your partner's or closest friend's mother's maiden name? Even the most dedicated person would have difficulty guessing the answer.