The best way to prevent access to any online account is to use a strong password. Use both upper and lower case letters, numbers and symbols, and make the password at least 8 characters long. Do not use any real words as the basis for the password. Note that if someone does guess your password they will likely be able to access your email from another computer.
Note also it is important to secure access to your computer. If security is a primary concern, do not use automatic login. Set a good password for login as well. Even better set a firmware password. You should also secure physical access to the computer. Keep it in your posession or locked up at all times.
Change your passwords frequently so that even if they are guessed, no one will have access for long.
Best of luck.