Every diner or restaurant has its own variation on the Monte Cristo; in fact, I've never seen a monte cristo like the one WolfDaddy describes. (mayonnaise?!?!) Here I attempt to give an idea of the range of variation. My monte cristos have mostly been eaten in Pennsylvania and New Jersey - maybe the mayo is a California thing?
Choose some or all of the following:
Sometimes the french toast is prepared ahead of time and later added to the sandwich; at other times, the sandwich as a whole may be cooked, something like a grilled cheese sandwich (but with egg, of course; also, I hear that sometimes it may be fried in a light batter coating). Two slices of french toast may be used, or sometimes three. Turkey and ham are standard; bacon is less common. One or two cheeses may be used.
My favorite so far is from the Edison Diner in New Jersey - turkey, ham, american, swiss, french toast, and raspberry jam. Some days, if the cook is in a good mood, it will also be served with powdered sugar.