The key is modularity. This is easier to design, implement, compile and maintain.
- Identify modules in your app, like classes in an OO app.
- Separate interface and implementation for each module, put in interface only what is needed by other modules. Remember that there is no namespace in C, so you have to make everything in your interfaces unique (e.g., with a prefix).
- Hide global variables in implementation and use accessor functions for read/write.
- Don’t think in terms of inheritance, but in terms of composition. As a general rule, don’t try to mimic C++ in C, this would be very difficult to read and maintain.
If you have time for learning, take a look at how an Ada app is structured, with its mandatory package (module interface) and package body (module implementation).
This is for coding.
For maintaining (remember that you code once, but you maintain several times) I suggest to document your code; Doxygen is a nice choice for me. I suggest also to build a strong regression test suite, which allows you to refactor.