Password protect your computer. It's simple, easy and will thwart casual threats.
Similarly, put a PIN lock on your mobile phone.
Just imagine if one of these items was stolen?
The thief can go to any part of the computer or phone that you do: email, web history, log in to websites with saved passwords, look through documents, pictures, address book....everything.
It's not fool-proof, but it adds a simple layer of security.
Do not under any circumstances use the same password for your online banking as other sites. If the other sites get hacked and your password is exposed, they bad guys will have your online banking password too.
Use a password manager to store your passwords, and make a backup in case you lose the data. With a password manager, you can use seriously strong, tough to guess (and remember) passwords, that can be stored inside the password manager for easy retrieval.
We like the password application 1Password for the Mac.
We also like GRC's Perfect Passwords. Just be careful how you use them! We suggest keeping them in 1Password.