What are the most common naming conventions in C?

The most important thing here is consistency. That said, I follow the GTK+ coding convention, which can be summarized as follows:

1. All macros and constants in caps: MAX_BUFFER_SIZE, TRACKING_ID_PREFIX.
2. Struct names and typedef’s in camelcase: GtkWidget, TrackingOrder.
3. Functions that operate on structs: classic C style: gtk_widget_show(), tracking_order_process().
4. Pointers: nothing fancy here:
   GtkWidget *foo, TrackingOrder *bar.
5. Global variables: just don’t use global variables. They are evil.
6. Functions that are there, but
   shouldn’t be called directly, or have
   obscure uses, or whatever: one or more
   underscores at the beginning:
   _refrobnicate_data_tables(), _destroy_cache().