All values that are used repeatedly should be constants or resources. Small items (mainly numerics, chars, and some very short strings) used repeatedly should be made constants, either "#define"d or "const". Strings of four or more characters, or other large-storage items, should be stored as "const"s or resources, rather than #defined, as #define does not save any storage space.
All constants, whether "const" or "#define"d, should be in UPPERCASE to denote this status.
Items (including functions) repeated across many projects should be extracted to a company-wide common library, including its own .h file.