Look at the Goddard Space Flight Center (NASA) C coding standard (at this URL). It has some good and interesting guidelines.
One specific guideline, which I've adopted for my own code, is that headers should be self-contained. That is, you should be able to write:
#include "header.h"
and the code should compile correctly, with any other necessary headers included, regardless of what has gone before. The simple way to ensure this is to include the header in the implementation source -- as the first header. If that compiles, the header is self-contained. If it doesn't compile, fix things so that it does. Of course, this also requires you to ensure that headers are idempotent - work the same regardless of how often they are included. There's a standard idiom for that, too:
#ifndef HEADER_H_INCLUDED
#define HEADER_H_INCLUDED
...operational body of header.h...
#endif /* HEADER_H_INCLUDED */
It is imperative, of course, to have the #define at the top of the file, not at the bottom. Otherwise, if a header included by this also includes header.h, then you end up with an infinite loop - not healthy. Even if you decide to go with a strategy of:
#ifndef HEADER_H_INCLUDED
#include "header.h"
#endif /* HEADER_H_INCLUDED */
in the code that include the header - a practice which is not recommended - it is important to include the guards in the header itself too.
Update 2011-05-01
The GSFC URL above no longer works. You can find more information in the answers for the question Should I use #include in headers, which also contains a cross-reference to this question.
Update 2012-03-24
The referenced NASA C coding standard can be accessed and downloaded via the Internet archive:
http://web.archive.org/web/20090412090730/http://software.gsfc.nasa.gov/assetsbytype.cfm?TypeAsset=Standard
