open up your script editor in maya.
go to Edit > Clear All. then paste the following to the bottom section of the script editor (Its called Input work area)


// Toggle Xray
//

string $visPanel[]=`getPanel -vis`;
int $mode;
if (size($visPanel)>0)
{
for ($panel in $visPanel)
{
if (`match "^modelPanel" $panel`=="modelPanel")
{
$mode=`modelEditor -q -xray $panel`;
if ($mode>0)
modelEditor -e -xray 0 $panel;
else
modelEditor -e -xray 1 $panel;
}
}
print ("\nxray="+$mode+".");
} else
print ("\nNo model panels to modify.");


after pasting it. highlight the whole code (ctrl + A) then go to File > Save Selected to Shelf, give a name to it. then it'll add the mel script onto the shelf that is active.

Enjoy