Open up the script editor, do menu->Edit->Clear All
Paste the below code in, highlight it with the mouse and MMB drag it onto the shelf. Voila. This script is great for toggling X-Ray on and off.

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// This toggles Xray on current Panel. Rating: 10/10
// Author: Luis Cataldi
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if (`getPanel -to (eval("getPanel -withFocus"))`)
{
$currentPanel = `getPanel -withFocus`;
if (`modelEditor -q -xray $currentPanel`)
{
modelEditor -edit -xray 0 $currentPanel;
}
else
{
modelEditor -edit -xray 1 $currentPanel;
}
};