The scanning quality is very important. You should obtain a quality comparable with the quality of the sample file shipped with SimpleOCR.
Usually you should use a scanning resolution of 300 DPI. It could be less if the characters are quite big or more if the characters are small.
Next, carefully tune the scanning brightness. Look at the characters in the resulting image. They should be clean. If you have a lot of characters in several pieces (i.e. image is too light) or many characters stuck together (i.e. image is too dark), you should tune the scanning brightness (or try a higher scanning resolution).
If your document has a sophisticated page layout, you can help SimpleOCR by selecting the text areas by hand using the “Create Area” tool.
Avoid having a scanning area bigger than the document. In this case you can have black borders around the document and SimpleOCR doesn’t like that.