Good way to organize C source files?
The way I've always organized my C source was to put struct, macro and function prototypes in header files and function implementations in .c files. However, I have recently been reading alot of other peoples code for large projects and I'm starting to see that people often define things like structs and macros in the C source itself, immediately above the functions that make use of it. I can see some benefit to this as you don't have to go searching around to find the definition of structs and macros used by particular functions, everything is right there in roughly the same place as the functions that use it. However I can also see some disadvantages to it as it means that there is not one central repository for struct/macro definitions as they're scattered through the sourcecode. 

My question is, what are some good rules of thumb for deciding when to put the macro/struct definition in the C source code as opposed to the header files themselves? 
Good way to organize C source files?
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The way I've always organized my C source was to put struct, macro and function prototypes in header files and function implementations in .c files. However, I have recently been reading alot of other peoples code for large projects and I'm starting to see that people often define things like structs and macros in the C source itself, immediately above the functions that make use of it. I can see some benefit to this as you don't have to go searching around to find the definition of structs and macros used by particular functions, everything is right there in roughly the same place as the functions that use it. However I can also see some disadvantages to it as it means that there is not one central repository for struct/macro definitions as they're scattered through the sourcecode. 

My question is, what are some good rules of thumb for deciding when to put the macro/struct definition in the C source code as opposed to the header files themselves? 