One of the best ways to create near photo-realistic effects using a vector drawing program such as Illustrator is to use a gradient mesh. This is a special kind of gradient that allows for multiple point of color fades between multiple color points. The regular gradient tools allow for multiple colors, but only from one side to another. . There can't be multiple point of color derivatives.
The series of leaves below was made using this tool. Notice the multiple point shading in the leaves. This is more noticeable in the blue ovals to the right of the leaves.
Meshes are only available in Illustrator 8.0 and above. This is a tool that should be used wisely. Gradient meshes can cause all sorts of printing issues. The later versions of Postscript printers usually have no trouble with meshes, but versions 2.0 and earlier can cause the printer to choke.
If gradient meshes are too complex to print, a simple Pantone blend or CMYK Process gradient will suffice.
