Beg or borrow a "Ringflash" and see if that is an improvement. They are quite expensive, so I wouldn't buy one without experimenting first.
Still not certain if flash is the right thing. I would have thought you would have got better control on this by using a couple of static lamps (of the anglepoise variety) so that you can get better control. Perhaps try a set of extension tubes if you are using a conventional reflex rather than a fixed lens digital.
I would go for fixed lights, no flash, small aperture and long exposure on a tripod to give you the control and depth of field you require.