• Bring up customization dialog with "Tools\Operations..."
• Bring up the keystroke mapper with "Keyboard..."
• Set focus in "Press Shortcut Keys" text box
• Press F7 or some other keystroke you want
• The "Shortcut currently used by" box will show the command currently invoked by the keystroke.
• Set focus to "Show Commands Containing" and type ToggleDesigner
• This will select the View.ToggleDesigner command in the list box of commands
• Next press the "Assign" button to assign the keystroke shortcut to the command.
• Click "OK" to the keystroke mapper dialog
• Click "Close" to the customization dialog

F7 - Visual Studio - switch between aspx and "codebehind"