USE YOUR LAST SETTINGS AND SAVE TIME
This is a tip that will save you time when you're making tonal adjustments using Levels, Curves, Color Balance, etc. (most anything that appears under the Adjustments menu under the Image menu). When you bring up one of the tonal adjustment dialogs, it always displays its default settings, but if you hold the Option key (PC: Alt key) when choosing it from the Adjustments menu, instead of coming up with the default settings, it will display the last settings you used in that particular dialog. You can also add the Option (PC: Alt) key to the keyboard shortcuts. For example, the shortcut to bring up the Levels dialog is Command-L (PC: Control-L), but if you add Option (PC: Alt) to those keys, the Levels dialog will open with your last-used settings.
Although there's no way to set a default size for your New document dialog (outside of creating an action to do it for you), there's a pretty cool little-known tip that lets you open a new document using the last settings you manually input. Just press Option-Command-N (PC: Alt-Control-N) and the New document dialog will appear using the last settings you input (rather than using the clipboard data to determine the new document size).
OPENING AN IMAGE AND CLOSING THE FILE BROWSER
Want to open multiple photos at once and have Photoshop close the File Browser for you? Just Command-click (PC: Control-click) on all the photos you want to open, then press Option-Return (PC: Alt-Enter). All the selected photos will open, and the File Browser will close.