In a large mixing bowl, combine the popcorn, pretzels, m&ms and any other things you prefer (raisinets, nuts, etc)
In a medium saucepan over medium heat, melt and combine the brown sugar, margarine and corn syrup (stirring constantly)
Once that mixture is combined and just starting to boil, add in the marshmallows (the marshmallows are the secret ingredient! I’ve never seen them in a caramel sauce before!)
Melt the marshmallows down and once the mixture is boiling, let it boil for about 2 minutes, stirring constantly.
The trick is to never stop stirring!
When the 2 minutes is up, pour the caramel mixture over the popcorn/pretzel/m&m mixture.
Stir well with a nonstick spoon (you may spray it with pam), and make sure all the popcorn mixture is covered.
Serve immediately or with in an hour or 2. Popcorn and pretzels can go stale very quickly.