static char associated_obj_key;
id anObj;

NSMutableArray *a = [[NSMutableArray alloc] init];
objc_setAssociatedObject(anObj, &associated_obj_key, a, OBJC_ASSOCIATION_RETAIN);
[a release];
	
- (NSMutableArray*) getOrCreateArray 
{ 
    static const char key;
    NSMutableArray* arr = objc_getAssociatedObject(self, &key); 
    if (!arr) 
    { 
        arr = [NSMutableArray array];
        objc_setAssociatedObject(self, &key, arr, OBJC_ASSOCIATION_RETAIN);
    }
    return arr;
}

- (void) someMethod
{
    id something = // ....
    [[self getOrCreateArray] addObject: something];
}