A sandwich is merely a combination of food items, usually always including 2 slices of bread and a some sort of meat product in the middle of them. A Sammich is an event. A sammich may or may not be life changing. A sammich is deserving of a lot of hard work to make and a special nickname when it's complete, such as 'Sexwich' or 'Deathwich'.
If any sandwich is arousing or almost kills you because of its sweet, sweet goodness, it is a sammich.
If you spend more than 10 minutes making a sandwich, and you do so with special care and love and lots of ingredients, it is most likely a sammich.
Anything with more than 3 layers is definitely a sammich.
And finally, if it's the best sandwich you've ever had (Like the Quiznos turkey-bacon-guacamole sub) it's safe to call it a sammich.