fix DPI (if needed) 300 DPI is minimum
fix text size (e.g. 12 pt should be ok)
try to fix text lines (deskew and dewarp text)
try to fix illumination of image (e.g. no dark part of image
binarize and de-noise image

http://stackoverflow.com/questions/9480013/image-processing-to-improve-tesseract-ocr-accuracy
http://en.wikipedia.org/wiki/Image_scaling

User Scenario: Process digital camera pictures and OCR to extract specific numbers

https://mazira.com/blog/optimal-image-conversion-settings-tesseract-ocr