file.c

// Getting Loaded
// Source: http://googlemac.blogspot.com/2006/11/getting-loaded.html

@implementation Foo (FooBarAdditions)
...
@end

@interface Foo (FooBarAdditionsPrivateMethods)
+ (void) initializeBar;
@end

@implementation Foo (FooBarAdditionsPrivateMethods)
+ (void) initializeBar {
    // Initialize stuff here
}
@end

void __attribute__ ((constructor)) InitializeFooBar(void) {
    static BOOL wasInitialized = NO; 
    // safety in case we get called twice.
    if (!wasInitialized) {    
        [Foo initializeBar];
        wasInitialized = YES;
    }
}