best way to suppress "unused" warning?

I'm using an API which does a lot of callbacks. In classic callback 
style, each routine provides a void * pointer to carry user-defined 
data. Sometimes, however, the user-defined pointer is not needed which 
causes the compiler to spit out a "helpful" warning. For example:

% cat unused.c
#include <stdio.h>
int
foo(char *str, void *data)
{
     puts(str);
     return 0;
}

% gcc -c -W -Wall unused.c
unused.c:4: warning: unused parameter 'data'

I'm looking for an appropriate (safe, reasonably self-do***enting) way 
of suppressing that warning in cases where I expect it (thus I'm not 
looking for the compiler flags to suppress the warning - I just want to 
be able to mark the places I know about). I've used constructs like

	data = data;
	data++;
	if (data != data) return;

And they all work but are a little too obfuscated for my taste. I'm sure 
many other people have run into this - is there a recommended technique?

RM