Two bits of advice I have are: 1) take the picture from as nearly perpendicular to the surface as you can and 2) try to ensure roughly even lighting if possible. Both of these can be corrected for to some degree. You can use the perspective correction tools (PS has one) to even out some perspective issues but it's easier to do with a nearly perpendicular image...I think it might be tough with your example photos because of the surface detail you're capturing. Hope that helps.
M