I don't rock bibs. But I often rock a pair of baggy shorts over the spandex that I'm rocking. When I do that, I rock a belt and tighten it enough so that when I pull up the spandex, it doesn't fall down. If you rock a fanny pack, it would have the same effect.

In the winter, when I'm rocking more than one layer, I'll often rock a pair of supsenders between the base layer that I'm rocking and my jersey. However, I've also found that since I've been rocking a Brooks saddle, what I rock when I ride my bike is less of an issue.