Avoiding declaring private functions in class header files (C++)

(In C++) I have a class whose structure is declared in a header file. That header file is included in lots of source files, such that when I edit it I need to recompile lots of files.

The class has a set of private functions which are only called in one source file. Currently they are declared in the class structure in the header file. When I add a new function of this type, or edit the arguments, it therefore causes recompilation of lots of files. I would like to declare the functions somewhere else, such that only the file that defines and calls them is recompiled (to save time). They still need to be able to access the internal class variables, though.

How can I achieve this?