Future versions of Apache for Unix may well be multithreaded, and, of course, the Win32 version already is. If you want your module to stand the test of time, you should avoid global variables, if at all possible. If not possible, put some thought into how they will be used by a multithreaded server. Don't forget that you can use the notes table in the request record to store any per-request data you may need to pass between hooks.
Never use a fixed-length buffer. Many of the security holes found in Internet software have fixed-length buffers at their root. The pool mechanism provides a rich set of tools you can use to avoid the need for fixed-length buffers.
Remember that your module is just one of a random set an Apache user may configure into his or her server. Don't rely on anything that may be peculiar to your own setup. And don't do anything that might interfere with other modules (a tall order, we know, but do your best!).