Considerations when designing a file type

Question

I'm about to start writing a process for saving some data structure from code in to a file of some proprietary, as-yet-undefined type. However, I've never designed a file type or structure before.

• Are there any things, generally speaking, that I should consider before starting my design?
• Are there any accepted good practices here? Bad practices I should avoid?
• Any absolute do's and don'ts?