After you invoke Exposé by using either the Exposé key, F9, or F10, pressing the tab key will cycle through your open windows, one application at a time (all the Safari windows, then all the Mail windows, then... etc.). Keep pressing the tab key until you reach the application that you are looking for.
If you want to return to the previous application while Exposé is still activated, just press shift + tab. You can select a specific open window in the app by clicking with the mouse or using the left/right arrow keys.
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