Themes and presentation make all of the difference when marketing your chocolate fountain business. Do something fun and different to set yourself apart from the competition.

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Try a chocolate fountain s'more party. Take the traditional ingredients and "kick it up a notch".
Instead of bagged marshmallows, try serving a variety of flavored home made marshmallows like mint (perfect companion with chocolate) or strawberry.

For kids parties use Harry Potter style names for the flavors like grasshopper for green mint flavor. Don't be afraid to go over the top with names for the kids. A kid may go "ewww" over a plate of broccoli, but just tell them it's "boogar flavored" and most of them will at least try it.
Use your imagination and your themes will be what the guests remember in addition to your catering skills.

Use skewers to dip the marshmallows in the chocolate fondue and place them on a tea biscuit (instead of a graham cracker). My personal favorite is the Maria Tea Biscuits. They are a nice round crispy cookie, perfect for chocolate dipping and just the right size for a marshmallow.
You could also make bowls of flavored marshmallow fluff, spread it on the cookie and dip it in the chocolate fountain.

Do something special and your clients will remember and recommend you.