This method is useful for tracking the flow of control and amounts to the same as using TRACE macros in Visual C++.
Put a printf(...) at the beginning and end of each function, possibly at the beginning of loops and if and switch blocks.
The more adventurous out there might try using fprint(...) so that the messages don't interfere with the output on the screen, or so the information can persist.

On a formatting issue, I usually insert these statements at the very beginning of the line, ignoring indentation so that the debugging feature can be easily spotted as such in the source code because it is so out of place.