What methods are there to modularize C code?

Question

What methods, practices and conventions do you know of to modularize C code as a project grows in size?

Answer 1

1. Don't define variables in header files; instead, define the variable in the source file and add an extern statement (declaration) in the header. This will tie into #2 and #3.
2. Use an include guard on every header. This will save so many headaches.
3. Assuming you've done #1 and #2, include everything you need (but only what you need) for a certain file in that file. Don't depend on the order of how the compiler expands your include directives.