"A modal window is a secondary window that opens inside the main window. Users have to interact with it before they can return to the main window. Use modal windows when you have to take the user’s focus off the main window to an important window that needs their attention. -- UX Movement
Bask in its modal-y goodness
Blue button: User for Submit, Save, Ok
Red button: Delete only
Grey button: Everything else
For informative modals (ie, those that don't require any further user interaction) use an "Ok" button in the footer to close the modal, in addition to the close icon in the header. Example.
For confirmation modals (ie, confirming a user action, such as delete) where there is no additional information, center the buttons in the modal body. Example.
Bask in its modal-y goodness
You don't have to do anything else.