Standard copy lighting works well for illuminating most flat and 3D objects whose surface detail does not need to be highlighted.  Lights are mounted at 45 degrees to the lens axis and equidistant from the specimen.  This provides even, uniform lighting. 

It is often useful to place diffusion filters in front of the light sources to soften a 3D specimens shadows.  This is especially important when an object casts distracting shadows on itself or on the background.  The larger the surface area of the diffuser, the more it will soften the shadows.  I make my own diffusion filters with sheets of matte architects velum mounted in a foamcore frame.  Alternatively, you could purchase specialized “softbox” lightsources for quite a bit more money.