It is a biologically determined reaction for endotherms (so-called 'warm-blooded' animals), that is, when core body temperature decreases, the testes and penis retract toward the body in order to preserve heat. However, and probably due to tissue deformation, the penis can be made to hang lower even in periods of cold and physical exertion etc. There are two methods which come to mind that you could try to get a fuller flaccid: (a) DLD's exercises that Pandora linked to in an above post and,
taking vasodilators. Undoubtedly there are many more things you could try to in order to achieve a larger flaccid length and girth, but, these are what come immediately to mind.